Metrology system and method for determining a characteristic of one or more structures on a substrate

ABSTRACT

Described is a metrology system for determining a characteristic of interest relating to at least one structure on a substrate, and associated method. The metrology system comprises a processor being configured to computationally determine phase and amplitude information from a detected characteristic of scattered radiation having been reflected or scattered by the at least one structure as a result of illumination of said at least one structure with illumination radiation in a measurement acquisition, and use the determined phase and amplitude to determine the characteristic of interest.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/150,879, filed Oct. 3, 2018, which claims the benefit of EPApplication No. 17206967.6, filed Dec. 13, 2017, EP Application No.17199764.6, filed Nov. 2, 2017, and EP Application No. 17194905.0, filedOct. 5, 2017, which are hereby incorporated herein in their entiretiesby reference.

FIELD

The present invention relates to a metrology system or an inspectionsystem for determining a characteristic of structures on a substrate.The present invention also relates to a method for determining acharacteristic of structures on a substrate.

BACKGROUND

A lithographic apparatus is a machine constructed to apply a desiredpattern onto a substrate. A lithographic apparatus can be used, forexample, in the manufacture of integrated circuits (ICs). A lithographicapparatus may, for example, project a pattern (also often referred to as“design layout” or “design”) at a patterning device (e.g., a mask) ontoa layer of radiation-sensitive material (resist) provided on a substrate(e.g., a wafer).

To project a pattern on a substrate a lithographic apparatus may useelectromagnetic radiation. The wavelength of this radiation determinesthe minimum size of features which can be formed on the substrate.Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nmand 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet(EUV) radiation, having a wavelength within the range 4-20 nm, forexample 6.7 nm or 13.5 nm, may be used to form smaller features on asubstrate than a lithographic apparatus which uses, for example,radiation with a wavelength of 193 nm.

Low-k₁ lithography may be used to process features with dimensionssmaller than the classical resolution limit of a lithographic apparatus.In such process, the resolution formula may be expressed as CD=k₁×λ/NA,where λ is the wavelength of radiation employed, NA is the numericalaperture of the projection optics in the lithographic apparatus, CD isthe “critical dimension” (generally the smallest feature size printed,but in this case half-pitch) and k₁ is an empirical resolution factor.In general, the smaller k₁ the more difficult it becomes to reproducethe pattern on the substrate that resembles the shape and dimensionsplanned by a circuit designer in order to achieve particular electricalfunctionality and performance. To overcome these difficulties,sophisticated fine-tuning steps may be applied to the lithographicprojection apparatus and/or design layout. These include, for example,but not limited to, optimization of NA, customized illumination schemes,use of phase shifting patterning devices, various optimization of thedesign layout such as optical proximity correction (OPC, sometimes alsoreferred to as “optical and process correction”) in the design layout,or other methods generally defined as “resolution enhancementtechniques” (RET). Alternatively, tight control loops for controlling astability of the lithographic apparatus may be used to improvereproduction of the pattern at low k1.

In lithographic processes, it is desirable to make frequentlymeasurements of the structures created, e.g., for process control andverification. Various tools for making such measurements are known,including scanning electron microscopes or various forms of metrologyapparatuses, such as scatterometers. A general term to refer to suchtools may be metrology apparatuses or inspection apparatuses. Inspecific embodiments relatively small wavelength ranges of the visiblelight spectrum is used for the inspection of the structures manufacturedon the substrate and there is a need to move to higher and to lowerwavelengths and there is a need to use broader wavelength ranges duringa single measurement. In specific embodiments, the optical system of theinspection or metrology apparatus has a relative high NA. In specificembodiments of the optical system, stray light and/or ghosting may be aproblem. In specific embodiment dark-field imaging is used and theamount of recorded noise may be too high in certain cases. It may bepossible to provide solutions for the above discussed requirementsand/or to provide a solution for the above discussed problems byimproving the quality of the optical elements used in the inspection ormetrology apparatus. A problem is that, for example, aberrationrequirements over a wide wavelength range become very strict. This mayresult in optical elements that cannot be manufactured or may result inoptical elements that become too expensive for the metrology orinspection apparatus.

SUMMARY

It is an object to provide an effective an efficient solution for aninspection or metrology system or apparatus that solves one or more ofthe above discussed problems or limitations.

Embodiments of the invention are disclosed in the claims and in thedetailed description.

In a first aspect of the invention there is provided a metrology systemfor determining a characteristic of interest relating to at least onestructure on a substrate, the metrology apparatus comprising: aprocessor being configured to: computationally determine phase andamplitude information from a detected characteristic of scatteredradiation having been reflected or scattered by the at least onestructure as a result of illumination of said at least one structurewith illumination radiation in a measurement acquisition, and use thedetermined phase and amplitude to determine the characteristic ofinterest.

In a second aspect of the invention there is provided a method ofdetermining a characteristic of interest relating to at least onestructure on a substrate, method comprising: computationally determiningphase and amplitude information from a detected characteristic ofscattered radiation having been reflected or scattered by the at leastone structure as a result of illumination of said at least one structurewith illumination radiation in a measurement acquisition, and using thedetermined phase and amplitude to determine the characteristic ofinterest

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings, in which:

FIG. 1 depicts a schematic overview of a lithographic apparatus;

FIG. 2 depicts a schematic overview of a lithographic cell;

FIG. 3 depicts a schematic representation of holistic lithography,representing a cooperation between three key technologies to optimizesemiconductor manufacturing;

FIG. 4 illustrates an inspection apparatus according to an embodiment ofthe invention;

FIG. 5 schematically depicts a metrology method using EUV radiation;

FIG. 6 schematically depicts an EUV metrology device according to anembodiment of the invention;

FIG. 7 depicts a known form of multiple grating target and an outline ofa measurement spot on a substrate;

FIG. 8 depicts an image of the target of FIG. 7 obtained by a metrologymethod such as that disclosed herein; and

FIG. 9 schematically depicts a metrology method according to anembodiment of the invention.

DETAILED DESCRIPTION

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) andEUV (extreme ultra-violet radiation, e.g. having a wavelength in therange of about 5-100 nm).

The term “reticle”, “mask” or “patterning device” as employed in thistext may be broadly interpreted as referring to a generic patterningdevice that can be used to endow an incoming radiation beam with apatterned cross-section, corresponding to a pattern that is to becreated in a target portion of the substrate. The term “light valve” canalso be used in this context. Besides the classic mask (transmissive orreflective, binary, phase-shifting, hybrid, etc.), examples of othersuch patterning devices include a programmable mirror array and aprogrammable LCD array.

FIG. 1 schematically depicts a lithographic apparatus LA. Thelithographic apparatus LA includes an illumination system (also referredto as illuminator) IL configured to condition a radiation beam B (e.g.,UV radiation, DUV radiation or EUV radiation), a mask support (e.g., amask table) MT constructed to support a patterning device (e.g., a mask)MA and connected to a first positioner PM configured to accuratelyposition the patterning device MA in accordance with certain parameters,a substrate support (e.g., a wafer table) WT constructed to hold asubstrate (e.g., a resist coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate support inaccordance with certain parameters, and a projection system (e.g., arefractive projection lens system) PS configured to project a patternimparted to the radiation beam B by patterning device MA onto a targetportion C (e.g., comprising one or more dies) of the substrate W.

In operation, the illumination system IL receives a radiation beam froma radiation source SO, e.g. via a beam delivery system BD. Theillumination system IL may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic,electrostatic, and/or other types of optical components, or anycombination thereof, for directing, shaping, and/or controllingradiation. The illuminator IL may be used to condition the radiationbeam B to have a desired spatial and angular intensity distribution inits cross section at a plane of the patterning device MA.

The term “projection system” PS used herein should be broadlyinterpreted as encompassing various types of projection system,including refractive, reflective, catadioptric, anamorphic, magnetic,electromagnetic and/or electrostatic optical systems, or any combinationthereof, as appropriate for the exposure radiation being used, and/orfor other factors such as the use of an immersion liquid or the use of avacuum. Any use of the term “projection lens” herein may be consideredas synonymous with the more general term “projection system” PS.

The lithographic apparatus LA may be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system PS and the substrate W—which is also referred to asimmersion lithography. More information on immersion techniques is givenin U.S. Pat. No. 6,952,253, which is incorporated herein by reference.

The lithographic apparatus LA may also be of a type having two or moresubstrate supports WT (also named “dual stage”). In such “multiplestage” machine, the substrate supports WT may be used in parallel,and/or steps in preparation of a subsequent exposure of the substrate Wmay be carried out on the substrate W located on one of the substratesupport WT while another substrate W on the other substrate support WTis being used for exposing a pattern on the other substrate W.

In addition to the substrate support WT, the lithographic apparatus LAmay comprise a measurement stage. The measurement stage is arranged tohold a sensor and/or a cleaning device. The sensor may be arranged tomeasure a property of the projection system PS or a property of theradiation beam B. The measurement stage may hold multiple sensors. Thecleaning device may be arranged to clean part of the lithographicapparatus, for example a part of the projection system PS or a part of asystem that provides the immersion liquid. The measurement stage maymove beneath the projection system PS when the substrate support WT isaway from the projection system PS.

In operation, the radiation beam B is incident on the patterning device,e.g. mask, MA which is held on the mask support MT, and is patterned bythe pattern (design layout) present on patterning device MA. Havingtraversed the mask MA, the radiation beam B passes through theprojection system PS, which focuses the beam onto a target portion C ofthe substrate W. With the aid of the second positioner PW and a positionmeasurement system IF, the substrate support WT can be moved accurately,e.g., so as to position different target portions C in the path of theradiation beam B at a focused and aligned position. Similarly, the firstpositioner PM and possibly another position sensor (which is notexplicitly depicted in FIG. 1) may be used to accurately position thepatterning device MA with respect to the path of the radiation beam B.Patterning device MA and substrate W may be aligned using mask alignmentmarks M1, M2 and substrate alignment marks P1, P2. Although thesubstrate alignment marks P1, P2 as illustrated occupy dedicated targetportions, they may be located in spaces between target portions.Substrate alignment marks P1, P2 are known as scribe-lane alignmentmarks when these are located between the target portions C.

As shown in FIG. 2 the lithographic apparatus LA may form part of alithographic cell LC, also sometimes referred to as a lithocell or(litho)cluster, which often also includes apparatus to perform pre- andpost-exposure processes on a substrate W. Conventionally these includespin coaters SC to deposit resist layers, developers DE to developexposed resist, chill plates CH and bake plates BK, e.g. forconditioning the temperature of substrates W e.g. for conditioningsolvents in the resist layers. A substrate handler, or robot, RO picksup substrates W from input/output ports I/O1, I/O2, moves them betweenthe different process apparatus and delivers the substrates W to theloading bay LB of the lithographic apparatus LA. The devices in thelithocell, which are often also collectively referred to as the track,are typically under the control of a track control unit TCU that initself may be controlled by a supervisory control system SCS, which mayalso control the lithographic apparatus LA, e.g. via lithography controlunit LACU.

In order for the substrates W exposed by the lithographic apparatus LAto be exposed correctly and consistently, it is desirable to inspectsubstrates to measure properties of patterned structures, such asoverlay errors between subsequent layers, line thicknesses, criticaldimensions (CD), etc. For this purpose, inspection tools (not shown) maybe included in the lithocell LC. If errors are detected, adjustments,for example, may be made to exposures of subsequent substrates or toother processing steps that are to be performed on the substrates W,especially if the inspection is done before other substrates W of thesame batch or lot are still to be exposed or processed.

An inspection apparatus, which may also be referred to as a metrologyapparatus, is used to determine properties of the substrates W, and inparticular, how properties of different substrates W vary or howproperties associated with different layers of the same substrate W varyfrom layer to layer. The inspection apparatus may alternatively beconstructed to identify defects on the substrate W and may, for example,be part of the lithocell LC, or may be integrated into the lithographicapparatus LA, or may even be a stand-alone device. The inspectionapparatus may measure the properties on a latent image (image in aresist layer after the exposure), or on a semi-latent image (image in aresist layer after a post-exposure bake step PEB), or on a developedresist image (in which the exposed or unexposed parts of the resist havebeen removed), or even on an etched image (after a pattern transfer stepsuch as etching).

Typically the patterning process in a lithographic apparatus LA is oneof the most critical steps in the processing which requires highaccuracy of dimensioning and placement of structures on the substrate W.To ensure this high accuracy, three systems may be combined in a socalled “holistic” control environment as schematically depicted in FIG.3. One of these systems is the lithographic apparatus LA which is(virtually) connected to a metrology tool MT (a second system) and to acomputer system CL (a third system). The key of such “holistic”environment is to optimize the cooperation between these three systemsto enhance the overall process window and provide tight control loops toensure that the patterning performed by the lithographic apparatus LAstays within a process window. The process window defines a range ofprocess parameters (e.g. dose, focus, overlay) within which a specificmanufacturing process yields a defined result (e.g. a functionalsemiconductor device)—typically within which the process parameters inthe lithographic process or patterning process are allowed to vary.

The computer system CL may use (part of) the design layout to bepatterned to predict which resolution enhancement techniques to use andto perform computational lithography simulations and calculations todetermine which mask layout and lithographic apparatus settings achievethe largest overall process window of the patterning process (depictedin FIG. 3 by the double arrow in the first scale SC1). Typically, theresolution enhancement techniques are arranged to match the patterningpossibilities of the lithographic apparatus LA. The computer system CLmay also be used to detect where within the process window thelithographic apparatus LA is currently operating (e.g. using input fromthe metrology tool MT) to predict whether defects may be present due toe.g. sub-optimal processing (depicted in FIG. 3 by the arrow pointing“0” in the second scale SC2).

The metrology tool MT may provide input to the computer system CL toenable accurate simulations and predictions, and may provide feedback tothe lithographic apparatus LA to identify possible drifts, e.g. in acalibration status of the lithographic apparatus LA (depicted in FIG. 3by the multiple arrows in the third scale SC3).

In lithographic processes, it is desirable to make frequentlymeasurements of the structures created, e.g., for process control andverification. Various tools for making such measurements are known,including scanning electron microscopes or various forms of metrologyapparatuses, such as scatterometers. Examples of known scatterometersoften rely on provision of dedicated metrology targets, such asunderfilled targets (a target, in the form of a simple grating oroverlapping gratings in different layers, that is large enough that ameasurement beam generates a spot that is smaller than the grating) oroverfilled targets (whereby the illumination spot partially orcompletely contains the target). Further, the use of metrology tools,for example an angular resolved scatterometter illuminating anunderfilled target, such as a grating, allows the use of so-calledreconstruction methods where the properties of the grating can becalculated by simulating interaction of scattered radiation with amathematical model of the target structure and comparing the simulationresults with those of a measurement. Parameters of the model areadjusted until the simulated interaction produces a diffraction patternsimilar to that observed from the real target.

Scatterometers are versatile instruments which allow measurements of theparameters of a lithographic process by having a sensor in the pupil ora conjugate plane with the pupil of the objective of the scatterometer,measurements usually referred as pupil based measurements, or by havingthe sensor in the image plane or a plane conjugate with the image plane,in which case the measurements are usually referred as image or fieldbased measurements. Such scatterometers and the associated measurementtechniques are further described in patent applications US20100328655,US2011102753A1, US20120044470A, US20110249244, US20110026032 orEP1,628,164A, incorporated herein by reference in their entirety.Aforementioned scatterometers can measure in one image multiple targetsfrom multiple gratings using light from soft x-ray and visible tonear-IR wave range.

A metrology apparatus, such as a scatterometer, is depicted in FIG. 4.It comprises a broadband (white light) radiation projector 2 whichprojects radiation 5 onto a substrate W. The reflected or scatteredradiation 10 is passed to a spectrometer detector 4, which measures aspectrum 6 (i.e. a measurement of intensity I as a function ofwavelength λ) of the specular reflected radiation 10. From this data,the structure or profile 8 giving rise to the detected spectrum may bereconstructed by processing unit PU, e.g. by Rigorous Coupled WaveAnalysis and non-linear regression or by comparison with a library ofsimulated spectra. In general, for the reconstruction, the general formof the structure is known and some parameters are assumed from knowledgeof the process by which the structure was made, leaving only a fewparameters of the structure to be determined from the scatterometrydata. Such a scatterometer may be configured as a normal-incidencescatterometer or an oblique-incidence scatterometer.

EUV Spectroscopic Reflectometry

FIG. 5 illustrates an EUV metrology method while FIG. 6 illustrates anEUV metrology apparatus 300. The apparatus can be used as an example ofEUV metrology apparatus 244 for measuring parameters of substrates Wprocessed in the manufacturing system of FIG. 1. The illuminationradiation used by EUV metrology apparatus may comprise radiation in thewavelength range from 0.1 to 100 nm, or, optionally, in the wavelengthrange from 1 to 100 nm or, optionally, in the wavelength range from 1 to50 nm, or optionally, in the wavelength range from 10 to 20 nm.

In FIG. 5, the target T is represented schematically as comprising aone-dimensional grating structure at the origin of a spherical referenceframe. Axes X, Y and Z are defined relative to the target. (Of courseany arbitrary coordinate system can be defined in principle, and eachcomponent may have its own local reference frame, that can be definedrelative to the one shown.) The direction of periodicity D of the targetstructure is aligned with the X axis. The drawing is not a trueperspective drawing, but a schematic illustration only. The X-Y plane isthe plane of the target and substrate, and for clarity is shown tiltedtoward the viewer, represented by an oblique view of circle 302. The Zdirection defines the direction N normal to the substrate. In FIG. 5,one of the incident rays is labeled 304 and has an angle α of grazingincidence. In this example, the incident ray 304 (and all incident raysforming the radiation spot S) lie substantially in a plane parallel tothe X-Z plane, that is a plane defined the directions D and N andrepresented by circle 306. A reflected ray 308 that is not scattered bythe periodic structure of the target T emerges towards the right handside of the target in the diagram, with an elevation angle α.

To perform spectroscopic reflectometry, ray 308 and other reflected raysare broken into a spectrum 310, comprising rays of differentwavelengths. The spectrum may be produced for example using a grazingincidence diffraction grating 312. The spectrum is detected by aspectrum detector 313. This spectrum detector 313, which may for examplebe a CCD image detector having an array of pixels, is used to transformthe spectrum into electrical signals and eventually digital data foranalysis.

In addition to spectrum 310, higher (non-zero) diffraction orders 352(e.g., at least the +1 and −1 orders, and possibly other higher orders)may be detected using a diffraction order detector 350. While onediffraction order detector 350 is shown here, more than one higher orderdetector may be used; for example, a first higher order detector for the+1 order, and a second higher order detector for the −1 order.Diffraction order detector 350 may for example be a CCD image detectorhaving an array of pixels.

In a practical system, the spectrum of radiation 304 may be subject totime variations, which would disturb the analysis. In order to normalizethe detected spectrum 310 and/or higher diffraction orders 352 againstthese variations, a reference spectrum is captured by a referencespectrum detector 314. To produce the reference spectrum, sourceradiation 316 is diffracted by another diffraction grating 318. A zeroorder reflected ray of grating 318 forms the incident ray 304, while thefirst order diffracted rays 320 of grating 318 form the referencespectrum detected by reference spectrum detector 314. Electrical signalsand data representing the reference spectrum are obtained for use in theanalysis.

From the measured spectrum 310 and/or higher diffraction orders 352,obtained for one or more values of incidence angle α, a measurement of aproperty of the target structure T can be calculated in a mannerdescribed further below.

Turning to FIG. 6, EUV metrology apparatus 300 is provided for measuringproperties of a metrology target T formed on substrate W, by the methodof FIG. 5. Various hardware components are represented schematically.The practical implementation of these components can be performed by therelevant skilled persons applying a mixture of existing components andspecially-designed components, according to well-known designprinciples. A support (not shown in detail) is provided for holding thesubstrate at a desired position and orientation relative to othercomponents to be described. A radiation source 330 provides radiation toan illumination system 332. Illumination system 332 provides a beam ofEUV illumination radiation represented by ray 304 which forms a focusedirradiation spot on target T. Illumination system 332 also provides thereference spectrum 320 to reference spectrum detector 314. Components312, 313 etc. may be conveniently considered as a spectrum detectionsystem 333.

Substrate W in this example is mounted on a movable support having apositioning system 334 such that an angle of incidence a of ray 304 canbe adjusted and/or the x, y, z position of the substrate W can beadjusted. In this example, it is chosen as a matter of convenience totilt the substrate W to change the incidence angle, while the source 330and illumination system 332 remain stationary. In order to catch thereflected ray 308, detection system 333 is provided with a furthermovable support 336, so that it moves through an angle 2 a relative tothe stationary illumination system, or through an angle α relative tothe substrate. In the grazing incidence regime of reflectometry, it isconvenient to define the incidence angle α by reference to the plane ofthe substrate, as shown. Of course, it could equally be defined as anangle between the direction of incidence of incident ray I and adirection N normal to the substrate.

Additional actuators, not shown, are provided for bringing each target Tinto a position where the focused spot S of radiation is located.(Looking at it another way, to bring the spot to the position where thetarget is located.) In a practical application, there may be asuccession of individual targets or target locations to be measured on asingle substrate, and a succession of substrates too. It is immaterial,in principle, whether the substrate and target are moved and reorientedwhile the illumination system and detectors 313, 350 stay still, orwhether the substrate stays still while the illumination system anddetectors 313, 350 are moved, or whether different components of therelative movement are achieved by a combination of these techniques. Thepresent disclosure encompasses all these variants.

As already described with reference to FIG. 5, the radiation reflectedby target T and substrate W is split into a spectrum 310 of rays ofdifferent wavelengths, before it impinges on spectrum detector 313.Spectrum detector 313 and/or diffraction order detector 350 comprisesfor example a position-sensitive EUV detector, typically an array ofdetector elements. In each case, the array may be a linear array, but inpractice a 2-dimensional array of elements (pixels) may be provided.Spectrum detector 313 and/or diffraction order detector 350 may be forexample a CCD (charge coupled device) image sensor.

A processor 340 receives signals from the detectors 350, 313 and 314. Inparticular, signal ST from spectrum detector 313 represents the targetspectrum, signals SF from diffraction order detector 350 represents thehigher order diffraction patterns and signal SR from detector 314represents the reference spectrum. Processor 340 can subtract thereference spectrum from the target spectrum to obtain a reflectionspectrum of the target, normalized against variation in the sourcespectrum. The resulting reflection spectra for one or more angles ofincidence are used in the processor to calculate a measurement ofproperty of the target, for example CD or overlay. Similarly, Processor340 can subtract the reference spectrum from the higher diffractionorder patterns (spectra) 352 to obtain higher order diffraction patternswhich are normalized against variation in the source spectrum. Thesehigher diffraction order patterns 352 can be compared in intensityasymmetry measurements to calculate a measurement of property of thetarget, for example overlay or focus.

In practice, radiation from source 330 may be provided in a series ofshort pulses and signals SR and ST may be captured together for eachpulse. Difference signals for each individual pulse are calculated,before being aggregated into an overall reflection spectrum for thistarget at this angle of incidence. In this way, instability of thesource spectrum between pulses is corrected for. The pulse rate may bethousands, or even tens of thousands per second (hertz). The number ofpulses aggregated to measure one reflection spectrum may be tens orhundreds, for example. Even with so many pulses, the physicalmeasurement takes a fraction of one second.

Applying this EUV spectroscopic reflectometry to metrology insemiconductor manufacturing, small grating metrology targets can beused. Multiple diffraction spectra are captured using detectors 350, 313and 314, while setting the grazing angle of incidence a to variousdifferent values. Using the spectra detected by spectrum detector 313and a mathematical model of the target structure, reconstructioncalculations can be performed to arrive at measurement of CD and/orother parameters of interest. Alternatively or in addition,complementary higher diffraction orders detected by diffraction orderdetector 350 may be compared to determine asymmetry in the targetstructure, and therefore one or more related parameters of interest suchas overlay, focus or dose, depending on the target properties.

In one embodiment of the scatterometer MT, the scatterometer MT isadapted to measure the overlay of two misaligned gratings or periodicstructures by measuring asymmetry in the reflected spectrum and/or thedetection configuration, the asymmetry being related to the extent ofthe overlay. A similar method may be used to measure focus on specialtargets which are formed with a focus dependent asymmetry. In theoverlay case, the two (typically overlapping) grating structures may beapplied in two different layers (not necessarily consecutive layers),and may be formed substantially at the same position on the wafer. Thescatterometer may have a symmetrical detection configuration asdescribed e.g. in co-owned patent application EP1,628,164A, such thatany asymmetry is clearly distinguishable. This provides astraightforward way to measure misalignment in gratings. Furtherexamples for measuring overlay error between the two layers containingperiodic structures as target is measured through asymmetry of theperiodic structures may be found in PCT patent application publicationno. WO 2011/012624 or US patent application US 20160161863, incorporatedherein by reference in its entirety.

FIG. 7 shows an exemplary metrology target T on a substrate W, such aswhich may be used to measure overlay. The target T may comprise anensemble of composite gratings or sub-targets 32, 33, 34, 35, formed bya lithographic process, mostly in resist, but also after an etch processfor example. For overlay applications, the sub-targets 32, 33, 34, 35may a pair of similar gratings (same pitch, CD, SWA, etc.) formed on thesame position on the wafer in different, but not necessarilyconsecutive, layers. The metrology apparatus will measure themisalignment between these two overlapping gratings, known as overlaymeasurement. In an embodiment, the target T may be designed for darkfield measurement, using a suitable scatterometer. A dark field targetwill typically be made smaller than the available illumination spot 31(typical target is 5×5 microns squared whereas an illumination spot hasa diameter of 35 micrometers). As such there will be sufficient space touse multiple overlay sub-targets 32, 33, 34, 35 which can be measured atthe same time, allowing measurements of multiple functionalities.Sub-targets 32, 33, 34, 35, may differ in their orientation, as shown,so as to diffract incoming radiation in X and Y directions. In thespecific example illustrated, sub-targets 32 and 34 are X-directionsub-targets with biases of +d, −d, respectively, and sub-targets 33 and35 are Y-direction sub-targets with offsets +d and −d respectively.Alternatively, measurement in only one direction would require only halfthe sub-targets, i.e., only those corresponding to that direction. Whilefour sub-targets are illustrated, another embodiment may include alarger matrix to obtain desired accuracy. For example, a 3×3 array ofnine composite sub-targets may have biases −4d, −3d, −2d, −d, 0, +d,+2d, +3d, +4d. Separate images of these sub-targets can be identified inthe image captured by detection system.

In an embodiment, asymmetry of the target, and hence, e.g., overlay, canthen be determined. This may be done using an image processor and acontroller by comparing the intensity values obtained for +1 and −1orders (or other complementary higher orders) for each periodicstructure 32-35 to identify any difference in their intensity, i.e., anintensity asymmetry. The term “difference” is not intended to refer onlyto subtraction. Differences may be calculated in ratio form or in sumform. The measured intensity asymmetries for a number of periodicstructures are used together with, if applicable, knowledge of theoverlay biases of those periodic structures to calculate one or moreperformance parameters of the lithographic process in the vicinity ofthe target T. A performance parameter of interest is overlay. Otherparameters of performance of the lithographic process can be calculatedsuch as focus and/or dose. The one or more performance parameters can befed back for improvement of the lithographic process, used to improvethe measurement and calculation process of the scatterometer itself andused to improve the design of the target T, for example.

More specifically, using for example the method described in PCT patentapplication publication no. WO 2011/012624 or US patent application US20160161863, incorporated herein by reference in its entirety, overlaybetween the two layers containing the sub-targets 32 to 35 may bemeasured by a method comprising the following steps. In an initial step,the substrate, for example a semiconductor wafer, is processed throughthe lithographic cell of FIG. 2 one or more times, to create a structureincluding the target comprising periodic structures 32-35. In a nextstep, a first diffraction pattern from the periodic structures 32 to 35is obtained using one of the first order diffracted beams (say −1). Inan embodiment, a first illumination mode is used. Then, whether by, forexample, changing the illumination mode, or changing the imaging mode,or by rotating substrate W by 180° in the field of view of the metrologyapparatus, a second diffraction pattern from the periodic structuresusing the other first order diffracted beam +1 is obtained.Consequently, the +1 diffracted radiation is captured in the secondimage. In an embodiment, the illuminated mode is changed and a secondillumination mode is used. In an embodiment, tool-induced artifacts likeTIS (Tool Induced Shift) can be removed by doing the measurement at 0°and 180° substrate orientation. The first and second diffractionpatterns are then compared, e.g., by calculating the differences ofintensity levels within diffraction patterns of each sub-target.

FIG. 8 shows an example of an image that may be formed on and detectedby a sensor, using the target of FIG. 7 as part of the method describedabove (e.g., using the apparatus of FIG. 6). The dark rectanglerepresents the field of the image on the sensor, within which theilluminated spot 31 on the substrate is imaged into a correspondingcircular area 41. Within this, rectangular areas 42-45 represent theimages of the small target gratings 32 to 35. If the targets are locatedin product areas, product features may also be visible in the peripheryof this image field. Image processor and controller PU processes theseimages using pattern recognition to identify the separate images 42 to45 of gratings 32 to 35. In this way, the images do not have to bealigned very precisely at a specific location within the sensor frame,which greatly improves throughput of the measuring apparatus as a whole.Once the separate images of the gratings have been identified, theintensities of those individual images can be measured, e.g., byaveraging or summing selected pixel intensity values within theidentified areas. Intensities and/or other properties of the images canbe compared with one another. These results can be combined to measuredifferent parameters of the lithographic process. Overlay performance isan important example of such a parameter.

Embodiments of the invention, which will be discussed hereinafter, maybe implemented in one of the above discussed metrology or inspectionapparatuses. Embodiments of the invention relate to methods and/or ametrology or inspection apparatus for determining a characteristic ofinterest of at least one structure (e.g., a target) on a substrate.

In present metrology apparatuses, a spatially incoherent light source istypically used. To increase the photon flux (in order to reduce themeasurement time), it would be desirable to use an (at least partially)coherent light source, such as white light laser, with an AOTF(Acousto-Optical Tunable Filter) to select the measurement wavelength(s)and/or bandwidth(s). A coherent illumination source might also allow fora small illumination spot, which is also beneficial (e.g., in order tosupport smaller target sizes or to prevent illumination of structuresneighboring the target). In addition, typically, in present metrologyapparatuses, the supported wavelength range is limited to approximately400 nm to 900 nm.

In general, the measurement of a suitably high quality intensity imageof requires that the optics should have low aberration specificationsover a wide wavelength λ range, such that there is flexibility to choosean optimal measurement wavelength and/or bandwidth. A wide wavelengthrange will enable measurements which cover a large number of differentmaterials, stacks and applications. At the same time, the optics shouldalso have a large numerical aperture NA (e.g., NA>0.8) to minimizecrosstalk between neighboring structures and a large field of view.Other considerations are a large dynamic range (lowghosting/reflections) and compatibility with the dark field measurementprinciple which suppresses the zeroth order.

It is very difficult or impossible to implement all of theserequirements and desirable features in a single metrology apparatus, asmany of them will impose competing requirements on the sensor optics tomaintain sufficiently low aberration performance. In particular,increasing the wavelength range of the illumination radiationsignificantly beyond the present 400 nm to 900 nm, while meeting theother requirements, will worsen the aberration performance of the sensoroptics. This will result in increased aberrations, which in turn willworsen the detector intensity image quality.

In particular, it is desirable to facilitate a larger wavelength range,for example 200 nm to 2000 nm), in combination with a large field ofview (>50 μm). Rather than attempt to achieve this optically whilemaintaining aberration performance, it is proposed that this be achievedby allowing the sensor optics to have greater aberrations. Of course,simply allowing larger aberrations within the sensor optics will have anunacceptable impact on the image quality unless something is done tocompensate for the effect of these optical aberrations. Therefore, it isproposed to use computational imaging techniques to compensate for thenegative effect of relaxation on aberration performance within thesensor optics.

As such, it is proposed to use computational imaging in a metrologyapparatus for measuring a structure on a substrate formed using alithographic process.

It is to be noted that instead of metrology apparatus one may also readmetrology system. For example, it is not necessary that the measurementsand the image processing are executed in the same apparatus. A separateimage processing apparatus may be coupled to a metrology apparatus toform a metrology system. It is also to be noted that instead ofmetrology apparatus or metrology system one may read inspectionapparatus or inspection system. E.g. the inspection apparatus thatcomprises an embodiment of the invention may be used to detect defectsof a substrate or defects of structures on a substrate. In such anembodiment, a characteristic of the structure on the substrate mayrelate to defects in the structure, the absence of a specific part ofthe structure, or the presence of an unwanted structure on thesubstrate.

Also proposed, in an optional embodiment, is a metrology apparatuscomprising an optical sensor layout in which separate optics for theillumination and detection branch, in order to reduce ghosting, straylight and/or reflection.

More specifically, it is proposed that a direct measurement of a targetdetector intensity image (i.e., an image of the illuminated targetcomprising the amplitude/intensity of the electric field at thedetector) is replaced by a phase retrieval, based on a (e.g., relativelylower quality) intensity measurement, which describes the interaction oftarget and illumination radiation in terms of electric field amplitudeand phase. This description may comprise a representation of theelectric and/or magnetic field at, for example, a plane immediatelyabove the target. In such an embodiment, the illuminated target electricand/or magnetic field image is modelled as an equivalent sourcedescription by means of infinitesimal electric and/or magnetic currentdipoles on a (e.g., two-dimensional) surface in a plane parallel withthe target. Such a plane may, for example be a plane immediately abovethe target, e.g., a plane which is in focus according to the Rayleighcriterion. It should be noted that present metrology apparatuses detectan intensity image at a conjugate plane with the plane immediately aboveor within the target. However the location of the model plane is notcritical: once amplitude and phase at one plane are known, they can bepropagated forward or backward in time to any other plane (e.g. infocus, out of focus, or even the pupil plane). The plane chosen may belocated before the (e.g. non-ideal, aberrating) optics such that, in thereimaging step which follows the phase retrieval algorithm, the targetcan be computationally reimaged under preferred circumstances (e.g.,assuming ideal sensor optics, and/or (almost) infinite numericalaperture, and/or specific (partial) coherence, and/or optimized pupil(complex) transfer mask/function). Alternatively, the description maycomprise a complex transmission of the target or a two-dimensionalequivalent thereof.

Typically the measured targets have unity permeability and non-unitypermittivity. Hence, in an embodiment it is proposed to make use of onlyelectric current dipoles, and not of magnetic dipoles, to represent thecombination of the target and the illumination.

The proposed phase retrieval may be used to obtain additional targetinformation, e.g., not only intensity/amplitude but also phaseinformation and/or an extended wavelength range. Also, the proposedphase retrieval may be used to obtain better quality target informationwhich can be used, for example, to compute overlay or focus (e.g., usingexisting overlay or focus algorithms). The better quality targetinformation may relate to intensity/amplitude only, or to bothintensity/amplitude and phase. By way of example, this better qualitytarget information may result from correcting for aberrations in thesensor optics in the phase retrieval algorithm, and/or from exploitingprior knowledge of the target in the phase retrieval algorithm.

A (candidate) phase retrieval algorithm has been designed that can becombined with any optical sensor layout, as it only requires theevaluation of the forward optical model and the computation of thederivative thereof. More details of the design of this phase retrievalalgorithm will be subsequently described.

Alternatively, or in combination with a prior knowledge approach,diverse measurements may be made. To achieve diversity, the imagingsystem may be (slightly) altered between the measurements. An example ofa diversity measurement is through-focus stepping, i.e., by obtainingmeasurements at different focus positions. Alternative methods forintroducing diversity include, for example, using different illuminationwavelengths or a different wavelength range, modulating theillumination, or changing the angle of incidence of the illumination onthe target between measurements.

In an embodiment, the methods described herein, e.g., more specificallythe phase retrieval by performance of a suitable algorithm and thedetermination of a characteristic of interest, may be performed on aprocessor forming part of a metrology apparatus. As such, the existingimaging detection optics of a metrology apparatus may be replaced withnew/other detection optics, a phase retrieval algorithm, and optionallya computational reimaging of the reconstructed electric field (bothamplitude and phase) to a detector intensity and/or phase image.

FIG. 9 is a flowchart conceptually describing a method as disclosedherein. FIG. 9 shows a metrology apparatus 900 comprising a sensoroptics and detector 910. As already described, the sensor optics mayhave increased capabilities (e.g., large wavelengthrange/field-of-view/NA) at the expense of aberration performance. Duringa measurement acquisition, an image 920 (e.g., of a target) is capturedon detector 910. However, this image 920 will be of insufficient qualitydue to the aberrations in the sensor optics. A phase retrieval algorithm930 is used to determine amplitude A and phase φ of the electric fieldat a plane parallel with the target (e.g., immediately above thetarget). Using a forward model of an ideal sensor (e.g., aberrations andcoherence), the target is reimaged 940 to obtain intensity I (inaddition to phase φ) of the field at the plane of detector 910. Notarget model is required. A suitable algorithm 950 is then used tocalculate the characteristic of interest 960 in the conventional manner.For example, overlay may be calculated from intensity asymmetry(intensity differences) in normal and complementary (positive andnegative) higher diffraction order pairs.

The phase retrieval algorithm can be combined with any of the followingthree optical sensor/detector layouts:

-   -   Measurements of a target where the detector is located in/near        an image plane (also referred to as a field plane).        -   This is likely to require the use of an objective lens, as            without it (i.e., using free space propagation) the detector            would need to be located unpractically close to the target            (e.g., a target-to-detector distance smaller than 100 μm for            a large field of view).    -   Measurements of a target where the detector is located in/near a        pupil plane (also referred to as a Fourier plane). This can be        implemented in two manners:        -   Using a lens between the target and detector to act as            detection optics.        -   Using free space propagation between the target and detector            (i.e., in a lensless configuration for example as            illustrated in FIGS. 5 and 6), whereby the detector is            located in/near the far-field.    -   Measurements of a target where the detector is located between        an image plane and a pupil plane.

Detectors may be placed in more than one of these locations such thatany combination of two or more of these layouts are possible, usingmultiple detectors each of which being located in a different plane.

It can be shown that locating the detector in either an image plane orin a pupil plane will result in a comparable photon shot noiseperformance on the measured target quadrant intensities for highnumerical aperture cases, when making use of a phase retrieval approachas described herein.

An advantage of locating the detector(s) in/near an image plane and/orpupil plane and/or in between an image plane and a pupil plane is thatmultiple target defocus (i.e., multiple focus settings) can be used toobtain more information of the same target (diversity measurements).This is (in principle) not possible when the detector is located in/nearthe far field (i.e., when only free space propagation is used in betweenthe target and detector).

To obtain diversity measurements at different focus settings(through-focus measurements), it is clear that the distance betweensample (target) and sensor can be varied between image captures. Thiscan be achieved by displacing the sample. However, this approach resultsin a large computational load as the light is digitally propagatedthrough the whole optical system to retrieve the full electric field atthe sample plane. To reduce the computational load, it is possible todisplace the sensor rather than the sample. In this manner, to retrievethe field at the detector plane, only free-space propagation between themeasurement planes is necessary. This phase retrieval can be done, forexample, by an iterative Fresnel-propagation based algorithm, or bytransport of intensity equations (TIE). This detector plane field is nowrequired to be digitally propagated only once through the setup toretrieve the light field at the object plane.

However, the required displacement of the sensor compared to thedisplacement of the target is much longer. This scales with the squareof the magnification of the optical system. This poses a clear speedlimitation: the sensor has to be translated over a long distance. Toaddress this, a further through-focus measurement setup is proposedwhich enables through focus measurements without mechanical displacementof the sample or sensor. In this setup, the focal length of the imagingsystem is tuned by electrical means. In this manner, the distancebetween the final optical element and the image plane is fixed. Apossible implementation uses an electrical tunable lens (ETL), which islocated on the low-NA side of the imaging system. Such ETLs have a fasttuning response enabling fast through-focus measurements. ETLs arecommercially available and, in addition to fast tuning, also have a hightransmission bandwith, low distortion and low costs.

For the lensless example, an advantage of locating the detector in/nearthe pupil plane is that, provided the distance between the target anddetector is large enough (e.g., ≥50 mm), the aberrations of thedetection optics can either be neglected, or if this distance is smaller(e.g., <50 mm), the aberrations are very well known and do not need tobe calibrated.

A detection optics aberration calibration could, for example, be basedupon measuring the same identical target for multiple different anglesof rotation (Rz) and/or position shifts (in the x- and y-direction). Asimilar approach can also be used to calibrate/characterize theillumination spot amplitude, phase and/or coherence distribution (e.g.,illumination speckle).

The target may be illuminated using spatially incoherent illuminationradiation (e.g., from a spatially incoherent source), using spatiallycoherent illumination radiation (e.g., from a spatially coherent source)or using spatially partially coherent illumination radiation (e.g., froma spatially partially coherent source). In the latter case usingpartially coherent illumination radiation, and where the detector isin/near an image plane, the effects thereof can be modelled by means ofa Sum of Coherent Systems approach. Should the detector be locatedin/near a pupil plane, Schell's theorem could provide an alternativemodelling approach which is computationally less demanding than for theSum of Coherent Systems approach.

It is typically desirable to maximize the ratio of wavelength to targetpitch (hereafter referred to as the λ/p ratio). However, as thenumerical aperture (NA) of the optics is increased, there is acorresponding increased likelihood of capturing orders higher than thefirst diffraction orders (e.g., a second diffraction order). In currentscatterometry systems, this will typically ruin the captured image. Theproposed methods described herein provide additional opportunities foreliminating the impact of the second order diffraction usingcomputational techniques.

A first proposal for minimizing the effects of second order diffractioncomprises using multiple low-NA lens and detector pairs, for exampleadjacent to each other, such that no two diffraction orders are capturedby the same detector. This can be implemented practically viacomputational imaging systems such as described herein, as the proposedoptics for such systems are relatively simple (e.g. detection opticscomprising one double aspherical lens between target and detector).

A further proposal for minimizing the effects of second orderdiffraction comprises making use of a phase retrieval algorithm asdescribed herein. Instead of retrieving only one amplitude and phaseimage of the target in the manner described, it is proposed tosimultaneously retrieve multiple images of the target, i.e., one imageper diffraction order. More specifically, it is proposed tosimultaneously retrieve one amplitude and phase image per diffractionorder which can propagate to the sensor. This approach is applicable tothe full vectorial case in addition to the scalar case.

As will be described in further detail below (in the section headed“Example phase retrieval algorithm”), a total variation inspiredregularization (i.e., prior knowledge) can be applied to amplitude andphase images corresponding to multiple illumination wavelengths and/orillumination polarizations. It is proposed that such regularization canalso be applied to the amplitude and phase images resultant frommultiple diffraction orders propagating to the sensor. It should benoted that propagating one phase and amplitude image per diffractionorder is equivalent to obtaining the electromagnetic light fieldradiated by the sample and a geometrical parameterization of the target,such that the field is subsequently divided into the diffraction ordersby (for example) a Fourier decomposition or the optical propagationitself.

It will be appreciated that the electromagnetic light field (intensityand phase) can also be obtained using a holographic setup.

Whether measured holographically or using computational phase retrieval,the target can be computationally re-imaged to introduce an arbitrary(e.g., chosen/optimized) pupil filtering/masking. In this way it ispossible to retrieve a (computationally) re-imaged sensed image of thetarget which comprises a chosen/optimized combination of diffractionorder information originating from the (retrieved) target.

Extending this further, it is possible to use the phase retrievalalgorithm to obtain information regarding the zeroth order and higherdiffraction orders. This can be done by either a) starting from a higherresolution image and performing suitable Fourier transformations(Fourier decomposition) and hence filter on the higher resolution image;or b) starting from separate images for each of the relevant diffractionorders and applying additional regularizations and/or constraints forthe phase retrieval algorithm as described (e.g., image similarity indifferent diffraction orders).

In an embodiment, any of the methods described herein may comprisecomputationally re-imaging the target electric field (for example, theelectric and/or magnetic electric field at target level, comprising acomplex target image) under different conditions, for example, fordifferent detection optics, aperture mask conditions and/or illuminationconditions. This applies whether the complex target image is measuredusing e.g., computational phase retrieval or measured directly usingholography.

A computationally optimized illumination condition may comprisecomputationally re-imaging with full incoherent illumination, forexample. This typically results in a smoother image with less ringingartifacts. This can be considered to be a form of coherence filteringwhich is likely beneficial to reduce the ringing effects present in thedetector intensity images, which can otherwise impact the measuredaverage intensities.

Such computational re-imaging techniques may also be used to applyapodization digitally. Presently, apodization may be applied by means ofa physical apodizer in (or near) a pupil plane of the imaging system forthe purpose of reducing crosstalk between structures being measured byengineering the (complex) point spread function of the imaging system.Such physical apodizers discard light and results in additionalreflections and losses in the beam path. A digital apodization algorithmalso has the advantage that it allows for dynamic tuning of theapodization, such that it can be adjusted for specifictarget/wafer/device structure combinations.

Additionally, the digital propagation and/or tuning of illuminationconditions might include optimization of:

-   -   the (digital) illumination coherence; assuming that the electric        fields at target level have full spatial coherence, partial        spatial coherence or are spatially incoherent. This can be        achieved, for example, by introducing an illuminator in the        re-imaging model. Alternatively it is possible to alter the        spatial coherence of the target directly (e.g. assume that        points in the target beyond a certain distance are mutually        incoherent for example). This is possible as the field at target        level is computationally retrieved, so in that sense there does        not need to be an explicit illuminator in the model when        retrieving the target.    -   (digital) illumination masking; i.e., blocking some regions of        the target illumination;    -   (digital) illumination spectrum shape;    -   (digital) illumination polarization; this requires separate        measurements for at least two different illumination        polarizations to be performed, or alternatively a very good        model of the illumination and target to be present;    -   (digital) polarizers in the (digital) optics to suppress the        above mentioned cross talk effects in the case of a        full-vectorial implementation (where there is access to the        polarization components of the electric field);    -   choose/optimize combinations of diffraction orders (as already        described); e.g., by optimizing pupil filtering/masking to        retrieve a derived (re-imaged) sensed image of the target which        comprises a chosen/optimized combination of diffraction order        information originating from the (retrieved) target;    -   optimize or change one or more conditions in the optical path        from the target to the sensor. Essentially, anything that can be        physically changed in an imaging system, can be digitally        changed in a digital model thereof; this may comprise for        example: changing optical details of the detection optics, e.g.,        changing the NA of the detection optics, changing any other        lenses/optical components (e.g., changing focal distance and/or        introducing/altering/removing aberrations), (digitally)        introducing a filter in detection branch;    -   choose/optimize different focus settings, e.g., re-imaging at        different focus values than the focus corresponding to the        actual measurement. Re-imaging to focus on different layers in        an image (e.g., obtained at a single focus setting).

It is also possible to average multiple images (i.e. average multipleimages computed by means of re-imaging as described above). In theremainder of this section, the computationally re-imaged images aredescribed as “re-images” for brevity. Such averaging may be useful, forexample, when measuring thick stacks, where the distance in between thetop and bottom grating of an overlay target might be large (e.g., withrespect to the depth of focus). In that case at least one of the edgesof the two gratings (i.e. top and bottom grating) is always blurry. Thisleads to process variations impacting the overlay performance. Toaddress this, it is proposed, for example, to compute a re-image that isat best focus for the top grating and another re-image that is at bestfocus for the bottom grating. These two re-images can then be averagedand overlay inferred (using asymmetry intensity for example) from theaveraged re-image, as the averaged re-image may be more process robust.

A further generalization of such averaging may comprise applying atechnique similar to optical color weighting (OCW), typically used inwafer alignment (OCW is described, for example, in Menchtchikov et al.“Reduction in overlay error from mark asymmetry using simulation, ORION,and alignment models” Proceedings of the SPIE, Volume 10587, id. 105870C10 pp. (2018); incorporated herein by reference). In such a proposal,two or more re-images are computed and combined using a weighting. Theweighting is such that the sum of weights equals one, but individualweights may be negative. Therefore the re-images are summed according tothe weight attributed to each re-image, with the sum of the weightsequaling one, to obtain a single weighted re-image. This makes itpossible to train the weights, such that the measurement becomes lesssensitive to process variations.

The weighting/averaging can be performed over the following dimensions:

-   -   Focus (as per the example above).    -   Wavelength.    -   Polarization.    -   Spatial coherence.    -   Pupil apodization/masking.    -   Diffraction order of target.

An advantage of weighting intensity re-images over weighting multipleoverlay estimates (i.e., one overlay estimate for each individualre-image), the latter example being most similar to what is done in OCWfor wafer alignment, is that it is possible to visually inspect/optimizefor favorable features in the averaged re-image. This is an advantage asno (external) absolute overlay reference is likely to be available fortraining to. An example of a favorable feature may be a minimalintensity ripple inside the overlay target region of interest.

Example Phase Retrieval Algorithm

Loss Function

In an embodiment, it is proposed that prior (target) knowledge isincluded in a loss function that forms the starting point toderive/design the phase retrieval algorithm. In such an embodiment, theprior knowledge may be based on certain observations; for example thereis much regularity between each image of the multiple images of thetarget. The multiple images may be obtained in a single measurement(e.g., a measurement using more than one illumination condition. e.g., amulti-wavelength measurement) or from diversity measurements (differentfocus levels etc.) already described. In this context, measuring underdifferent illumination conditions may comprise the use of illuminationradiation where one or more of wavelength and/or polarization and/ortemporal and/or spatial coherence of the illumination radiation isvaried between measurements. It can be observed that, regardless of theillumination condition/focus setting, each image comprises a flatstructure, essentially having the form of FIG. 8 (assuming a target ofthe form of FIG. 7). As such, each image can be observed to have thesame or very similar position and shape for each region of interest(e.g., 4 rectangular or square shaped ROIs, one in each quarter of alarger square or rectangular shaped target region), and where eachregion of interest comprises a region of relatively high intensityhaving a relatively flat intensity profile. It is proposed that thissimilarity between images be exploited, in an embodiment, by means of ageneralization of a Total Variation or Vector Total Variationregularization (i.e., imposing an L1 penalty on the gradient of thetarget image). A benefit of this Vector generalization is that itintroduces a coupling between different illumination conditions (thiscoupling hereafter referred to as wavelength coupling, although it ismore generally applicable to coupling of measurements corresponding tovariation of other illumination conditions). This wavelength couplingmay be similar to the coupling which results from the introduction ofdispersion models (also known as n&k models as they describe therelationship of refractive index n and/or extinction coefficient kthrough wavelength), but without the need for layer thicknesses, layercomplex permittivity information or geometrical parameterization of thetarget to be specified, for example.

It should be clear that diversity, prior target knowledge or both may beused in the phase retrieval. With sufficient diversity, it should not benecessary to impose a constraint and/or regularization based on priortarget knowledge to ensure convergence. Equally, by constraining and/orregularizing the phase retrieval algorithm using prior target knowledge,diversity (e.g., measurement at different levels of defocus) should notbe necessary. Greater accuracy or better guarantee of convergence may beobtained by using both diversity and imposing a prior target knowledgeconstraint and/or regularization.

The wavelength coupling resultant from the use of prior knowledge (e.g.,as a constraint, regularization or hybrid regularization and constraint)also can be used to suppress of coherent imaging artifacts such as imagespeckle. In normal imaging systems, a well-known way to suppress speckleis to increase the bandwidth. It is proposed herein to perform the phasereconstruction for multiple wavelengths/illumination conditionssimultaneously. The resultant wavelength coupling introduces a form ofaveraging (which is what the proposed prior knowledge regularizationdoes). This leads to a computational equivalent to speckle reduction ina normal imaging system by increasing the bandwidth.

Performing the phase reconstruction for multiplewavelengths/illumination conditions also enables identification andremoval of coherent imaging artifacts which may result, in particular,from the proposed use of spatially coherent light. Such coherent imagingartifacts may comprise interaction between neighboring targets, such asringing artifacts and speckle (speckle is being classed as a coherentimaging artifact for the purposes of this disclosure; it is a relatedeffect, although originating from path length variations such as fromsurface roughness). Optics-induced ringing artifacts should bedistinguished from true processing effects which happen to resemble suchringing artifacts in an image. It can be observed that optics-inducedringing artifacts show a strong variation through wavelength, while trueprocessing effects do not. As such, the strong wavelength dependency canbe used in the image recovery algorithm to remove much of the effects ofoptics-induced ringing artifacts and speckles without suppressing trueprocessing effects, as the ringing-like effects resultant from trueprocessing effects have a spatial frequency that iswavelength-independent. One way of achieving this is by using theproposed vector generalization of a Total Variation or Vector TotalVariation regularization. The wavelengths can be chosen in a recipecreation setup, based on optimizing a range of KPIs such as sensitivity,stack sensitivity, process robustness and grating imbalance, forexample.

In phase retrieval there are in essence two possible algorithmicapproaches at the highest level. The first of these approaches is a“feasibility problem” or projection onto convex and/or non-convex setsapproach (e.g. the Gerchberg-Saxton algorithm) and the second approachis an optimization problem approach. As the proposed forward opticalmodel (see below) has no closed form inverse operator, the gradient ofthe non-linear forward optical model is used to implement the reversemapping. Such a gradient based minimization method may comprise, forexample, a gradient descent, Newton method, Gauss-Newton method orquasi-Newton method. Hence the optimization approach is moreappropriate, as a feasibility or projection based approach willultimately amount to an optimization problem when making use of thegradient of the non-linear forward optical model to implement thereverse mapping. It should also be appreciated that multiple detectorintensity measurements will be used in the proposed phase retrievalalgorithm.

It is proposed that the phase retrieval loss function has a datafidelity term and a prior information term (i.e., regularization term).Optionally the prior information may be used to apply one or moreconstraints to the phase retrieval minimization of the loss function. Inan embodiment, the data fidelity term comprises a least squares sum(i.e. an L2 norm) of the difference between the modeled intensities andthe measured intensities. An L2 norm is proposed because the noise ofthe measured intensities will have a near to Gaussian distribution, anda least squares minimization will result in a minimum variance unbiasedestimator. Additionally, a least squares minimization might lead topossibility, in future, to make use of a Gauss-Newton approximation ofthe resulting Newton equation (depending on the properties of theregularization term in the phase retrieval loss function).

For the prior information term (i.e., regularization term) of the phaseretrieval loss function, it is proposed to use an L1 norm, and morespecifically a total variation norm. The total variation norm is relatedto the contour length of the level set that represents the (unknown)target (phase and amplitude). As the unknown target consists of threeseparate polarization directions, and its measurement possibly alsorelates to more than one illumination condition, in an embodiment, avector extension of the total variation norm will be used. In thespecific case described herein, the unknown target is a complex function(i.e., not only intensity but both amplitude and phase), which impliesthat an additional extension of the vector total variation norm may besuitable. The additional extension proposed may be based on the factthat the dominant singular value of the Jacobian matrix equals thegradient magnitude for real vector fields. Making use of this vectorextension of the total variation norm will have the added benefit thatregistration errors (i.e., overlay) of the resulting target amplitudeand phase between illumination conditions will also be penalized. Notethat the prior information term of the phase retrieval loss functionwill act on a local target level (i.e., pixel neighborhood), which isbeneficial from an algorithm parallelization point of view.

In an embodiment, the phase retrieval loss function L may take the form:L=F _(D) +G _(T)  (1)The fidelity term F_(D), in a specific embodiment may take the form

$\begin{matrix}{F_{D} = {\frac{1}{2}{{\underset{\_}{R}}_{D}}_{2}^{2}}} & (2) \\{{\underset{\_}{R}}_{D} = {\begin{bmatrix}{{\underset{\_}{I}}_{D,{model}}(1)} \\{{\underset{\_}{I}}_{D,{model}}(2)} \\\vdots \\{{\underset{\_}{I}}_{D,{model}}(W)}\end{bmatrix} - \begin{bmatrix}{{\underset{\_}{I}}_{D,{meas}}(1)} \\{{\underset{\_}{I}}_{D,{meas}}(2)} \\\vdots \\{{\underset{\_}{I}}_{D,{meas}}(W)}\end{bmatrix}}} & (3) \\{{{\underset{\_}{I}}_{D,{model}}(w)} = {\sum\limits_{p \in {\{{x,y,z}\}}}\;\begin{bmatrix}{I_{D,{model}}\left( {{J_{T,p}\left( \lambda_{w} \right)},x_{w,1},y_{w,1},z_{w,1},\lambda_{w}} \right)} \\{I_{D,{model}}\left( {{J_{T,p}\left( \lambda_{w} \right)},x_{w,2},y_{w,2},z_{w,2},\lambda_{w}} \right)} \\\vdots \\{I_{D,{model}}\left( {{J_{T,p}\left( \lambda_{w} \right)},x_{w,S},y_{w,S},z_{w,S},\lambda_{w}} \right)}\end{bmatrix}}} & (4) \\{{{\underset{\_}{I}}_{D,{meas}}(w)} = \;\begin{bmatrix}{I_{D,{meas}}\left( {x_{w,1},y_{w,1},z_{w,1},\lambda_{w}} \right)} \\{I_{D,{meas}}\left( {x_{w,2},y_{w,2},z_{w,2},\lambda_{w}} \right)} \\\vdots \\{I_{D,{meas}}\left( {x_{w,{S{(w)}}},y_{w,{S{(w)}}},z_{w,{S{(w)}}},\lambda_{w}} \right)}\end{bmatrix}} & (5)\end{matrix}$The prior information term or regulation term in a specific embodimentmay take the form:

$\begin{matrix}{G_{T} = {\mu{\sum\limits_{u = 1}^{U}{g_{T}(u)}}}} & (6) \\{{g_{T}(u)} = {\sigma_{1}\left( \left\lbrack {{\partial_{x}{{\underset{\_}{\Gamma}}_{T}\left( {x_{u},y_{u}} \right)}}\mspace{20mu}{\partial_{y}{{\underset{\_}{\Gamma}}_{T}\left( {x_{u},y_{u}} \right)}}} \right\rbrack \right)}} & (7) \\{{{\underset{\_}{\Gamma}}_{T}\left( {x_{u},y_{u}} \right)} = \begin{bmatrix}{{\underset{\_}{J}}_{T}\left( {x_{u},y_{u},\lambda_{1}} \right)} \\{{\underset{\_}{J}}_{T}\left( {x_{u},y_{u},\lambda_{2}} \right)} \\\vdots \\{{\underset{\_}{J}}_{T}\left( {x_{u},y_{u},\lambda_{W}} \right)}\end{bmatrix}} & (8) \\{{{\underset{\_}{J}}_{T}\left( {x_{u},y_{u},\lambda_{w}} \right)} = \begin{bmatrix}{J_{T,x}\left( {x_{u},y_{u},\lambda_{w}} \right)} \\{J_{T,y}\left( {x_{u},y_{u},\lambda_{w}} \right)} \\{J_{T,z}\left( {x_{u},y_{u},\lambda_{w}} \right)}\end{bmatrix}} & (9)\end{matrix}$Where:

W denotes the total number of measured wavelengths;

S(w) denotes the total number of detector intensity sample positions forwavelength identifier w∈{1,2, . . . , W};

U denotes the total number of regularization target dipole electriccurrent sample positions;

λ_(w) denotes the measurement wavelength (or more generally,illumination condition) for wavelength identifier w∈{1,2, . . . , W};

J_(T,p) denotes the target dipoles electric current distribution, forthe three polarization directions p∈{x,y,z};

μ denotes the regularization multiplier, the value (≥0) of which is tobe chosen based on an appropriate level of regularization;

σ₁ (X) denotes the largest singular value of matrix X;

∂_(x) and ∂_(y) denote the partial derivative operator, in the targetx-direction respectively the target y-direction;

(x_(w,s), y_(w,s), z_(w,s)) for w∈{1, . . . , W} and s∈{1, . . . , S(w)}denote the intensity sample positions; and

(x_(u), y_(u), λ_(w)) for u∈{1, . . . , U} denote the target dipoleelectric current sample positions.

It should be noted that Equation (7) above gives only one embodiment ofthe Vector Total Variation norm. Another embodiment may comprise anuclear norm, i.e. g_(T)(u)=∥[∂_(x) Γ _(T)(x_(u),y_(u)) ∂_(y) Γ_(T)(x_(u),y_(u))]∥. In general any suitable scalar function of thesingular values and/or eigenvalues of the matrix within the squarebrackets may be used.

It should also be noted that the most dominant singular value of a realJacobian matrix (i.e., g_(T)(u) if it were a real matrix) equals thenon-negative gradient magnitude. Furthermore, in the implementation ofthe phase retrieval algorithm on a processor, it may be assumed that thetarget is periodic and discrete, thus enabling use of the fast Fouriertransform (FFT) to compute the Fourier transforms in the numericalevaluation of the forward optical model optical, and the use of a cyclicdifference matrix L to compute the derivatives ∂_(x) and ∂_(y) ofEquation (7).

Equations (4), (5), (6) and (7) will now be redefined into theirperiodic and discrete form as described by Equations (11), (12) and (13)below. These are a more practical form, which can be more efficientlysolved and therefore are the form proposed to be used herein, accordingto an embodiment:

$\;\begin{matrix}{\mspace{79mu}{{\overset{\Cup}{\underset{\_}{\underset{\_}{\Gamma}}}}_{T} = {\arg\;{\min\limits_{{\underset{\_}{\underset{\_}{\Gamma}}}_{T}}(L)}}}} & (10) \\{{{\underset{\_}{I}}_{D,{model}}(w)} = {\sum\limits_{p \in {\{{x,y,z}\}}}\begin{bmatrix}{I_{D,{model}}\left( {\left( {\underset{\_}{\underset{\_}{\Gamma}}}_{T} \right)_{({:{,{\{{n,{n + 1},{n + 2}}\}}}})},x_{w,1},y_{w,1},z_{w},\lambda_{w}} \right)} \\{I_{D,{model}}\left( {\left( {\underset{\_}{\underset{\_}{\Gamma}}}_{T} \right)_{({:{,{\{{n,{n + 1},{n + 2}}\}}}})},x_{w,2},y_{w,2},z_{w},\lambda_{w}} \right)} \\\vdots \\{I_{D,{model}}\left( {\left( {\underset{\_}{\underset{\_}{\Gamma}}}_{T} \right)_{({:{,{\{{n,{n + 1},{n + 2}}\}}}})},x_{w,{S{(w)}}},y_{w,{S{(w)}}},z_{w},\lambda_{w}} \right)}\end{bmatrix}}} & (11) \\{\mspace{85mu}{{{\underset{\_}{I}}_{D,{meas}}(w)}\begin{bmatrix}{I_{D,{meas}}\left( {x_{w,1},y_{w,1},z_{w},\lambda_{w}} \right)} \\{I_{D,{meas}}\left( {x_{w,2},y_{w,2},z_{w},\lambda_{w}} \right)} \\\vdots \\{I_{D,{meas}}\left( {x_{w,{S{(w)}}},y_{w,{S{(w)}}},z_{w},\lambda_{w}} \right)}\end{bmatrix}}} & (12) \\{\mspace{79mu}{{G_{T}\left( {\underset{\_}{\underset{\_}{L}} \cdot {\underset{\_}{\underset{\_}{\Gamma}}}_{T}} \right)} = {\sum\limits_{m = 1}^{UV}{\sigma_{1}\left( \left( {\underset{\_}{\underset{\_}{L}} \cdot {\underset{\_}{\underset{\_}{\Gamma}}}_{T}} \right)_{({{\{{m,{m + {UV}}}\}},:})} \right)}}}} & (13) \\{\mspace{79mu}{\underset{\_}{\underset{\_}{L}} = {\mu\begin{bmatrix}{\underset{\_}{\underset{\_}{D}}}_{x} \\{\underset{\_}{\underset{\_}{D}}}_{y}\end{bmatrix}}}} & (14) \\{\mspace{79mu}{{\underset{\_}{\underset{\_}{\Gamma}}}_{T} = {\mu\begin{bmatrix}{{\underset{\_}{\Gamma}}_{T}^{T}\left( {x_{1},y_{1}} \right)} \\{{\underset{\_}{\Gamma}}_{T}^{T}\left( {x_{2},y_{1}} \right)} \\\vdots \\{{\underset{\_}{\Gamma}}_{T}^{T}\left( {x_{U},y_{1}} \right)} \\{{\underset{\_}{\Gamma}}_{T}^{T}\left( {x_{1},y_{2}} \right)} \\\vdots \\{{\underset{\_}{\Gamma}}_{T}^{T}\left( {x_{U},y_{V}} \right)}\end{bmatrix}}}} & (15)\end{matrix}$where:

{hacek over (Γ)} _(T) denotes the retrieved phase and amplitude of thetarget;n=3w−2;

U now denotes the total number of regularization target dipole electriccurrent Cartesian sample positions in the x direction;

V notes the total number of regularization target dipole electriccurrent Cartesian sample positions in the y direction;

(x_(w,s), y_(w,s), z_(w,s)) for w∈{1, . . . , W} and s∈{1, . . . , S(w)}denote (a subset of) the detector intensity Cartesian sample positionssuch that it is possible to select all said Cartesian sample positionsor a subset thereof;

(X)_((:,n)) extracts column n of matrix X;

(X)_((m,:)) extracts row m of matrix X;

x ^(T) denotes the transpose of vector x; and

D _(x) and D _(y) denote the cyclic difference matrices which, for eachrow, have a −1 at the diagonal entry and 1 at the appropriateoff-diagonal entry, all other elements being zero;

Example Optimization Algorithms

The phase retrieval loss function as described by Equation (1) is aconvex function without continuous derivatives due to the L1regularization term. A simplified example of this is provided by theequation: |x|=σ₁([x 0]). Hence it is not advantageous to make use of agradient based optimization algorithm to implement Equation (10).Instead it is proposed to use a proximal based optimization algorithm toimplement Equation (10), as will be described in this section.

The optimization algorithm that may be used is a forward-backward basedprimal-dual algorithm. The term forward-backward applies to thesplitting of the loss function L into a first part having a continuousfirst order derivative; e.g., the data fidelity term F_(D) and a secondpart having a non-discontinuous first order derivative; e.g., the priorinformation term G_(T). As the gradient of the prior information termG_(T) does not exist, the gradient is replaced by the proximal operatorthereof, which leads to a forward step using the gradient of the datafidelity term F_(D) and a backward step using the proximal operator ofprior information term G_(T). The term “primal-dual” refers to the factthat the primal and dual optimization problem are simultaneously solved,which is beneficial as the proximal operator of G_(T) is easier tocompute in its dual form.

An example of an algorithm to minimize loss function L and thereforesolve Equation (10) above will now be provided. This algorithm iswritten in pseudo code using a notation that directly interfaces withthe loss function L (Equation (1)):

${1.\mspace{14mu}{Set}\mspace{14mu}{\underset{\_}{\underset{\_}{\Gamma}}}_{1}} \in {\mathbb{C}}^{{UV} \times 3W}$${2.\mspace{14mu}{Set}\mspace{14mu}{\underset{\_}{\underset{\_}{\Omega}}}_{1}} \in {\mathbb{C}}^{2\;{UV} \times 3W}$

3. Set τ∈]0,+∞[

4. Set ϕ∈]0,+∞[

5. k−1

6. while not converged do

${\left. {\left. {7.\mspace{14mu}{\underset{\_}{\underset{\_}{s}}}_{k}}\leftarrow{{\underset{\_}{\underset{\_}{\Gamma}}}_{k} - {\tau\frac{\partial F_{D}}{\partial_{\underset{\_}{\underset{\_}{\Gamma}}}}\left( {\underset{\_}{\underset{\_}{\Gamma}}}_{k} \right)}} \right.\left. {8.\mspace{14mu}{\underset{\_}{\underset{\_}{y}}}_{k}}\leftarrow{{\underset{\_}{\underset{\_}{s}}}_{k} - {\tau\;{{\underset{\_}{\underset{\_}{L}}}^{T} \cdot {\underset{\_}{\underset{\_}{\Omega}}}_{k}}}} \right.\left. {9.\mspace{11mu}{\underset{\_}{\underset{\_}{q}}}_{k}}\leftarrow{{prox}_{\phi\; G_{T}^{*}}\left( {{\underset{\_}{\underset{\_}{\Omega}}}_{k} + {\phi\;{\underset{\_}{\underset{\_}{L}} \cdot {\underset{\_}{\underset{\_}{y}}}_{k}}}} \right)} \right.\left. {10.\mspace{14mu}{\underset{\_}{\underset{\_}{p}}}_{k}}\leftarrow{{\underset{\_}{\underset{\_}{s}}}_{k} - {\tau\;{{\underset{\_}{\underset{\_}{L}}}^{T} \cdot {\underset{\_}{\underset{\_}{q}}}_{k}}}} \right.{{11.\mspace{14mu}{Set}\mspace{14mu}\eta_{k}} \in}} \right\rbrack 0},{+ {\infty\left\lbrack \left. {12.\mspace{14mu}{\underset{\_}{\underset{\_}{\Gamma}}}_{k + 1}}\leftarrow{{\underset{\_}{\underset{\_}{\Gamma}}}_{k} + {{\eta_{k}\left( {{\underset{\_}{\underset{\_}{p}}}_{k} - {\underset{\_}{\underset{\_}{\Gamma}}}_{k}} \right)}13.\mspace{14mu}{\underset{\_}{\underset{\_}{\Omega}}}_{k + 1}}}\leftarrow{{\underset{\_}{\underset{\_}{\Omega}}}_{k} + {{\eta_{k}\left( {{\underset{\_}{\underset{\_}{q}}}_{k} - {\underset{\_}{\underset{\_}{\Omega}}}_{k}} \right)}14.\mspace{14mu} k}}\leftarrow{k + {115.\mspace{14mu}{end}\mspace{14mu}{while}16.\mspace{14mu}{Return}\mspace{14mu}\overset{\Cup}{\underset{\_}{\underset{\_}{\Gamma}}}}}\leftarrow{{\underset{\_}{\underset{\_}{\Gamma}}}_{k + 1}17.\mspace{14mu}{Return}\mspace{14mu}\overset{\Cup}{\underset{\_}{\underset{\_}{\Omega}}}}\leftarrow{\underset{\_}{\underset{\_}{\Omega}}}_{k + 1} \right. \right.}}$where:

]a,b[={x∈

|a<x<b}

L ^(T) is the transpose of L

prox_(ϕG) _(T) _(*) (Ω _(T)) denotes the proximal operator of the scaledconjugate ϕG_(T)* (Ω _(T)); and

G_(T)* (Ω _(T)) denotes the conjugate of G_(T) (L·Γ _(T)). Note that theconjugate function is not the same as the complex conjugate. Note thatthe subscript _(T) of variables Γ _(T) and Ω _(T) has been omitted inthe above pseudo code description for clarity.

It should be noted that other unknowns may also be introduced asvariables; for example: an uncertain focus position of the target, anuncertain aberration function of the detection optics or an uncertainillumination property (e.g. illumination amplitude, phase, spatialcoherence and/or temporal coherence). This will simply result in thematrix L being padded with additional zeros.

Note that an iterative modification of matrix L is allowed (i.e. matrixL becomes a function of the iteration number k), this can be exploitedto further improve the estimation performance.

An advantage of this particular forward-backward based primal-dualalgorithm is that it requires the computation of the simpler proximaloperator prox

$\;_{\phi G_{T}^{\star}}\left( {\underset{\_}{\underset{\_}{\Omega}}}_{T} \right)$i.e., the left hand side multiplication with L is taken care of by theoptimization algorithm, and does not need to be taken into account inthe computation of the proximal operator itself. Another advantage isthat no inverse or decomposition of matrix L needs to be computed.

It can be shown that:

$\left( {pro{x_{\phi G_{T}^{*}}\left( {\underset{\_}{\underset{\_}{\Omega}}}_{T} \right)}} \right)_{({{\{{m,{m + {UV}}}\}},:})} = {\arg{\min\limits_{\underset{\_}{\underset{\_}{X}}}\left\{ {{l_{\mathcal{B}}\left( \underset{\_}{\underset{\_}{X}} \right)} + {\frac{1}{2\phi}{{\underset{\_}{\underset{\_}{X}} - \left( {\underset{\_}{\underset{\_}{\Omega}}}_{T} \right)_{({{\{{m,{m + {Uy}}}\}},:})}}}_{F}^{2}}} \right\}}}$$\mspace{79mu}{{l_{B}\left( \underset{\_}{\underset{\_}{X}} \right)} = \left\{ \begin{matrix}{{0\mspace{14mu}{if}\mspace{14mu}\underset{\_}{\underset{\_}{X}}} \in \mathcal{B}} \\{{\infty\mspace{14mu}{if}\mspace{14mu}\underset{\_}{\underset{\_}{X}}} \notin \mathcal{B}}\end{matrix} \right.}$where

={X|∥X∥_(*)≤1} denotes the matrix nuclear norm unit ball.

The projection of

$\left( {pro{x_{\phi G_{T}^{\star}}\left( {\underset{\_}{\underset{\_}{\Omega}}}_{T} \right)}} \right)_{({{\{{m,{m + {UV}}}\}},:})}$into the matrix nuclear norm unit ball

can be conveniently computed by means of a single value decomposition of

$\left( {\underset{\_}{\underset{\_}{\Omega}}}_{T} \right)_{({{\{{m,{m + {UV}}}\}},:})} \in$

^(2×3W). A suitable method for doing this can be found, for example, in:Goldluecke, Bastian, Evgeny Strekalovskiy, and Daniel Cremers; thenatural vectorial total variation which arises from geometric measuretheory; SIAM Journal on Imaging Sciences 5, no. 2 (2012): 537-563; (seecorollary 3.6, theorem 3.7 and FIG. 7), the entire document being hereinincorporated by reference. It should be noted that, in the above examplealgorithm, the matrix

$\left( {\underset{\_}{\underset{\_}{\Omega}}}_{T} \right)_{({{\{{m,{m + {UV}}}\}},:})}$is a complex matrix, while the equivalent matrix in the documentincorporated by reference in this paragraph is a real matrix. It will bereadily apparent to the skilled person how such a model may be adaptedfor a complex matrix.Other Regularization Options

The above described algorithm and cost function uses a specificregularization (i.e., prior information term G_(T)) to incorporate priortarget knowledge into the phase retrieval problem. However this is onlyan example of regularization and other methods to benefit from the prior(target) knowledge can be used. A number of such methods will now bedescribed. It should be noted that any of these prior forms ofregularization and/or “model order reduction” methods may be usedsingularly or in any combination, either as an alternative to the abovedescribed regularization or as one or more further constraints tosupplement this regularization. Different regularization methods, andcombinations thereof, may be better for measuring different structures(different stack and/or target properties) and in differentapplications.

A first possible option is to make use of a “minimum description length”(e.g., Kolmogorov complexity) based regularization. An exampleembodiment could compress the target electric field image by means ofLiv-Zempel and/or run-length like compression schemes and use the lengthof the resulting compressed bit stream as the metric. The compressionscheme may take advantage of the knowledge that the target structuresare simple, repetitive structures, and therefore the regularization mayimpose a bias against solutions with a high Kolmogorov complexity, forexample.

In a somewhat similar manner, a matrix/tensor rank based regularizationcould also be used, or an approximation of the matrix/tensor rank suchas the matrix nuclear norm (i.e. the sum of singular values of a matrixor tensor). Such a matrix rank or nuclear norm regularization may imposea low-rank constraint on the solution, for example. Again this will biasagainst solutions which imply complex target structures.

Another option, should measurement and/or simulation data of the targetbe available beforehand (i.e., before performing the actualmeasurement), this data can be exploited to generate (for example) asearch library, or to generate and exploit a projection into a lowerdimensional subspace. For example, the search library may be used toreduce the number of degrees of freedom, by limiting potential solutionsto only those which are a linear combination of library images of thetarget.

In another option, where hyperspectral illumination radiation is used(for example when measuring with 100 or more different illuminationwavelength and polarization combinations), a generic dispersion modelcould be used to reduce the total number of degrees of freedom.Determining a generic dispersion model may comprise modeling the changein refractive index (or similar parameter) againstwavelength/polarization (or propagation direction in the material) foreach target using the measurement data from each wavelength andpolarization combination. The model may then comprise a model of thetarget as a transmission function. The prior regularization functionwould, in such a case, operate on the input coefficients of such adispersion model. As the retrieved phase and amplitude iswavelength/polarization dependent, the model can output an image for awavelength and polarization combination of interest. In an embodiment,the electric permittivity function ε_(r) (ω) may take the form:

$\begin{matrix}{{ɛ_{r}(\omega)} = {1 + {\chi(\omega)}}} & (17) \\{{\chi(\omega)} = {c_{re} - {i \cdot {\sum\limits_{m = 0}^{M}\frac{a_{m}}{{i \cdot \omega} - b_{m}}}} - \frac{a_{m}^{*}}{{i \cdot \omega} - b_{m}^{*}}}} & (18) \\{b_{m} = {{{- \varsigma_{m}} \cdot \omega_{m}} + {\omega_{m} \cdot \sqrt{\varsigma_{m}^{2} - 1}}}} & (19)\end{matrix}$and c_(re) is the real offset constant, a_(m) is the dipole complexamplitude, ω_(m) dipole natural eigen-frequency, ζ_(m) is the dipolesampling constant

$\left( {0 < \varsigma_{m} < 1} \right)\mspace{14mu}{and}\mspace{14mu}\frac{1}{\varsigma_{m} \cdot \omega_{m}}$is the dipole time constant. Note that all poles occur as (conjugated)pole pairs (dipoles), where M denotes the total number of pole pairs.Example Optics Forward Model

The optics forward model used to calculate the modelled intensities inthe data fidelity term F_(D) of Equation (1) may take the form of avector forward model. An example coherent vector forward model will nowbe described. Other suitable models may also be used, for example themodel may be modified so that or can handle partially spatially coherentor spatially incoherent illumination radiation, as appropriate. In thedescription below, the subscripts D, P and T refer to the coordinatesystems of the detector, pupil and target respectively. Note that inthis geometry all lenses in between the target and the detector (via theaperture stop) have been replaced by a far field free space propagation.However, in an embodiment, the model may additionally model the effectof one or more optical elements (e.g., the objective lens) on thescattered radiation to obtain the modelled characteristic of thescattered radiation.

The vector forward model may begin with the observation that theelectric field on the entrance pupil is locally a plane wave andtherefore only its tangential electric field E components E_(P,ϑ) andE_(P,φ) are non-zero and the normal electric field component E_(P,ρ) iszero. As electric field at the exit pupil equals the electric field atthe entrance pupil, this property, also applies to the electric field atthe exit pupil.

It is convenient to compute the propagation of the electric field fromthe exit pupil to detector in the Cartesian polarization coordinatesystem, as in that case the direction of the individual polarizationcomponents does not depend on the spatial location at the detector.

$\begin{matrix}{\begin{bmatrix}{E_{D,x}\left( {x,y,z} \right)} \\{E_{D,y}\left( {x,y,z} \right)} \\{E_{D,z}(x,y,z)}\end{bmatrix} = {{\int{\int_{- \infty}^{+ \infty}{{{\Omega\left( {k_{x,},k_{y},\ z} \right)}\begin{bmatrix}{E_{P,x}\left( {k_{x,},k_{y}} \right)} \\{E_{P,y}\left( {k_{x,},k_{y}} \right)} \\{E_{P,z}\left( {k_{x,},k_{y}} \right)}\end{bmatrix}}{\exp\left( {{- i}{{\frac{2\pi}{\lambda}\ \begin{bmatrix}k_{x} \\k_{y}\end{bmatrix}}^{\dagger}.\ \begin{bmatrix}x \\y\end{bmatrix}}} \right)}dk_{x}d{k_{y}\mspace{79mu}\begin{bmatrix}k_{x} \\k_{y} \\k_{z}\end{bmatrix}}}}} = \begin{bmatrix}{\cos\;\varphi\;\sin\;\vartheta} \\{\sin\;\varphi\;\cos\;\vartheta} \\{\cos\;\vartheta}\end{bmatrix}}} & (20)\end{matrix}$Where

$\begin{matrix}{{\Omega\left( {k_{x,},k_{y},z} \right)} = {{D\left( {k_{x,},k_{y},z} \right)}{K\left( {k_{x,},k_{y}} \right)}{A\left( {k_{x,},k_{y}} \right)}}} & (21) \\{{D\left( {k_{x,},k_{y},z} \right)} = {\exp\left( {{- i}\frac{2\pi}{\lambda}k_{z}z} \right)}} & (22) \\{{A\left( {k_{x,},k_{y}} \right)} = \left\{ \begin{matrix}{{1\mspace{9mu}{for}\mspace{14mu} k_{r}} < {NA}} \\{{0\mspace{9mu}{or}\mspace{14mu} k_{r}} \geq {NA}}\end{matrix} \right.} & (23) \\{k_{r} = \sqrt{k_{x}^{2} + k_{y}^{2}}} & (24) \\{{K\left( {k_{x,},k_{y}} \right)} = {{\cos\;\vartheta} = {k_{z} = \sqrt{1 - k_{x}^{2} - k_{y}^{2}}}}} & (25)\end{matrix}$The detector intensity for the full coherent case I_(D,coh)(x, y, z) canbe calculated by:

$\begin{matrix}{{I_{D,{coh}}\left( {x,y,z} \right)} = {\begin{bmatrix}{E_{D,x}\left( {x,y,z} \right)} \\{E_{D,y}\left( {x,y,z} \right)} \\{E_{D,z}\left( {x,y,z} \right)}\end{bmatrix}^{\dagger} \cdot \begin{bmatrix}{E_{D,x}\left( {x,y,z} \right)} \\{E_{D,y}\left( {x,y,z} \right)} \\{E_{D,z}\left( {x,y,z} \right)}\end{bmatrix}}} & (26)\end{matrix}$where the dagger † denotes the combination of the transpose operator andthe complex conjugate operator.

In order to compute the electric field at the entrance pupil E _(P) thecombination/product of the illumination and target is modelled as aninfinitely dense array of infinitesimal electrical current dipoles withan electrical current distribution that equals J _(T) (x, y).Accordingly the Fourier transform relationship describing diffractionform the target to entrance pupil may take the form:

$\begin{matrix}{\begin{bmatrix}{E_{P,\rho}\left( {k_{x,},k_{y}} \right)} \\{E_{P,\vartheta}\left( {k_{x,},k_{y}} \right)} \\{E_{P,\varphi}\left( {k_{x,},k_{y}} \right)}\end{bmatrix} = {{\underset{\_}{\underset{\_}{\xi}}\left( {k_{x,},k_{y}} \right)}{\int{\int_{- \infty}^{+ \infty}{\begin{bmatrix}{J_{T,x}\left( {x,y} \right)} \\{J_{T,y}\left( {x,y} \right)} \\{J_{T,z}(x,y)}\end{bmatrix}{\exp\left( {i{{\frac{2\pi}{\lambda}\ \begin{bmatrix}k_{x} \\k_{y}\end{bmatrix}}^{\dagger} \cdot \begin{bmatrix}x \\y\end{bmatrix}}} \right)}{dxdy}}}}}} & (27) \\{\mspace{85mu}{{\underset{\_}{\underset{\_}{\xi}}\left( {k_{x,},k_{y}} \right)} = \begin{bmatrix}0 & 0 & 0 \\\frac{k_{x}k_{z}}{k_{r}} & \frac{k_{y}k_{z}}{k_{r}} & {- k_{r}} \\{- \frac{k_{y}}{k_{r}}} & \frac{k_{x}}{k_{r}} & 0\end{bmatrix}}} & \;\end{matrix}$Combining the above:

$\begin{matrix}{\begin{bmatrix}{E_{D,x}\left( {x,y,z} \right)} \\{E_{D,y}\left( {x,y,z} \right)} \\{E_{D,z}(x,y,z)}\end{bmatrix} = {\int{\int_{- \infty}^{+ \infty}{\begin{bmatrix}{E_{Q,x}\left( {k_{x,},k_{y},z} \right)} \\{E_{Q,y}\left( {k_{x,},k_{y},z} \right)} \\{E_{Q,z}\left( {k_{x,},k_{y},z} \right)}\end{bmatrix}{\exp\left( {{- i}{{\frac{2\pi}{\lambda}\ \begin{bmatrix}k_{x} \\k_{y}\end{bmatrix}}^{\dagger}.\ \begin{bmatrix}x \\y\end{bmatrix}}} \right)}dk_{x}dk_{y}}}}} & (28) \\{\begin{bmatrix}{E_{Q,x}\left( {k_{x,},k_{y},z} \right)} \\{E_{Q,y}\left( {k_{x,},k_{y},z} \right)} \\{E_{Q,z}\left( {k_{x,},k_{y},z} \right)}\end{bmatrix} = {{{{\underset{\_}{\underset{\_}{\Gamma}}\left( {k_{x,},k_{y}} \right)} \cdot {\Omega\left( {k_{x,},k_{y},\ z} \right)} \cdot {{\underset{\_}{\underset{\_}{\xi}}\left( {k_{x,},k_{y}} \right)}\begin{bmatrix}{J_{P,x}\left( {k_{x,},k_{y}} \right)} \\{J_{P,y}\left( {k_{x,},k_{y}} \right)} \\{J_{P,z}\left( {k_{x,},k_{y}} \right)}\end{bmatrix}}}{\underset{\_}{\underset{\_}{\Gamma}}\left( {k_{x,},k_{y}} \right)}} = {\quad\begin{bmatrix}k_{x} & \frac{k_{x}k_{z}}{k_{r}} & {- \frac{k_{y}}{k_{r}}} \\k_{y} & \frac{k_{y}k_{z}}{k_{r}} & \frac{k_{x}}{k_{r}} \\k_{z} & {- k_{r}} & 0\end{bmatrix}}}} & (29) \\{\begin{bmatrix}{J_{P,x}\left( {k_{x,},k_{y}} \right)} \\{J_{P,y}\left( {k_{x,},k_{y}} \right)} \\{J_{P,z}\left( {k_{x,},k_{y}} \right)}\end{bmatrix} = {\int{\int_{- \infty}^{+ \infty}{\begin{bmatrix}{J_{T,x}\left( {x,y} \right)} \\{J_{T,y}\left( {x,y} \right)} \\{J_{T,z}(x,y)}\end{bmatrix}{\exp\left( {i{{\frac{2\pi}{\lambda}\ \begin{bmatrix}k_{x} \\k_{y}\end{bmatrix}}^{\dagger}.\ \begin{bmatrix}x \\y\end{bmatrix}}} \right)}{dxdy}}}}} & (30)\end{matrix}$

Note that wavefront aberrations (transmission and/or phase) canadditionally also be included this expression. If they are applied inthe parallel, and a senkrecht polarization coordinate system is used forexample, then the aberration matrix function may be positioned inbetween the matrices Γ(k_(x) k_(y))·and·ξ(k_(x), k_(y)). Note that, incontrast to that assumed above, in general the aberration function is afour-dimensional function of both the pupil coordinates k_(x), and k_(y)and the spatial coordinates x and y.

Note that the matrix Γ is a unitary matrix and that the discrete Fouriertransform matrix is also a unitary matrix, and these two unitarymatrices commute. Hence in Equation (29) the multiplication with thematrix may be swapped in position with the inverse discrete Fouriertransform operation. Further, if the purpose is to numerically computethe detector intensity, see Equation (26), then the multiplication withmatrix may also be omitted, as the detector intensity can be computedusing the spherical polarization coordinate system.

SUMMARY

Above it is proposed to use computational imaging (e.g., phaseretrieval) in lithographic metrology applications (e.g., measuring acharacteristic or parameter of interest such as overlay, focus, dose orone or more geometrical parameter values such as critical dimension,side wall angle, edge placement error or local critical dimensionuniformity) on a target. Overlay may comprise a measure of misalignmentof structures in different layers and/or within-layer overlay, i.e., ameasure of misalignment of structures in a single layer, such as in adouble patterning process.

In an embodiment, it is proposed to use prior (target) knowledge inphase retrieval so as to enable a relaxation of the optics specification(e.g., aberration performance). In most present phase retrievalapplications, there is little (beforehand) knowledge of the subjectbeing measured (for example in biological applications), hence no (oronly limited) prior (target) knowledge can be exploited in the phaseretrieval algorithm. Also, in most known phase retrieval applicationsbright field measurements are typically used, and as such extending suchtechniques to dark field illumination is unknown. Applying a vectortotal variation based regularization function to one or more complexfields (i.e. amplitude and phase data instead of intensity data) ispresently unknown, as such functions are believed to have been onlyapplied to real fields (such as an intensity image) up till now. The oneor more complex fields may comprise a complex field for each electricdipole current component/orientation (i.e. x, y or z) per illuminationwavelength and polarization combination.

Advantages of the proposed methods include the following:

-   -   A wavelength coupling is provided, i.e., a coupling of        corresponding measurements of a target under different        illumination conditions. This wavelength coupling enables        exploitation of multi-wavelength (or multi-illumination        condition) measurements of one identical target (i.e. to couple        wavelengths), without the need to introduce dispersion models        (also known as n&k models) in μDBO (micro-diffraction based        overlay) applications. Such dispersion models describe the        relationship of refractive index n and/or extinction coefficient        k through wavelength. However, as described above, dispersion        models may optionally be used to reduce the number of degrees of        freedom.    -   The wavelength coupling can be used to suppress coherent imaging        artifacts such as image speckle or ringing artifacts by        introducing an averaging.    -   The phase retrieval algorithm specifically disclosed above has a        convergence guarantee (under certain suitable conditions not        further discussed here), unlike many other known phase retrieval        algorithms.    -   The fact that a target electric field replaces the detector        intensity image in the above disclosure allows for additional        advantages. This applies whether the target electric field is        measured using e.g., computational phase retrieval or measured        directly using holography. These additional advantages include:        -   Digital refocusing of the target is possible, after it has            been measured.        -   A target electric field can be computationally re-imaged            under different conditions using the methods described            above; the different conditions including inter alia:            different detection optics, aperture mask conditions and/or            illumination conditions.        -   In case of a thin resist target for example, the resulting            phase might provide a better source of overlay information            than the resulting amplitude/intensity.    -   Currently, it is possible to obtain a detector intensity image        for different wavelengths, bandwidths, illumination coherence        states and/or different illumination polarization states and/or        detector analyzer polarization states. It now also becomes        possible to introduce the following extra variations, which        allow to measure more/different information from one individual        target:        -   Defocus of the target (note that this is not possible when            the detector is located in a pupil plane).        -   Aberration and/or speckle and/or spatial coherence and/or            temporal coherence perturbations of the illumination spot.        -   Aberration perturbations of the detection optics.    -   In a typical metrology device sensor, a significant amount of        sensor optics is needed/used. The computational propagation of        the electric and/or magnetic field allows computational        generation of both a pupil image and field image without the        need to measure both. This allows for a reduction in the sensor        optics, and therefore may allow for volume and cost reduction        and/or allow for provision of multiple sensors in parallel, to        further reduce measurement time and/or increase sampling density        of a substrate.    -   The specific phase retrieval algorithm described above is well        suited to be implemented efficiently on a graphics processing        unit (GPU) for example. No linear system of equations needs to        be solved. The evaluation of the prior knowledge part of the        loss function can be done on a pixel to pixel basis, leading to        an embarrassingly parallel problem for that part of the loss        function. For the data fidelity part of the loss function, the        amount of computations will be dominated by computing FFTs (Fast        Fourier Transforms). Also the memory footprint of the phase        retrieval algorithm is not large.

Other embodiments are disclosed in the subsequent clauses:

i. A metrology apparatus for determining a characteristic of one or morestructures on a substrate, the metrology system comprising

a sensor to detecting characteristics of electromagnetic radiation beingreflected or scattered by the one or more structures,

a processor being configured to

receive the detected characteristics of the electromagnetic radiation,

determine phase and amplitude of an electric field in the vicinity ofthe one or more structures on the substrate,

using the determined phase and amplitude of the electric field todetermine the characteristic.

ii. A metrology system according to clause i, wherein the processor usesa feasibility problem approach, such as for example a Gerchberg-Saxtonalgorithm, to determine the phase and the amplitude.

iii. A metrology system according to clause i, wherein the processor isconfigured to determine the phase and amplitude by

a) defining an optimization problem based on the detectedcharacteristics of the electromagnetic radiation, a model of theinteraction of the radiation with the one or more structures andknowledge of the radiation impinging on the one or more structures atthe moment that the sensor detected the characteristics of theelectromagnetic radiation; andb) finding a solution for the optimization problem.iv. A metrology system according to clause iii, wherein the processor isfurther configured to use knowledge of the one or more structures on thesubstrate to determine the phase and amplitude.v. A metrology system according to clause iii or iv, whereinregularization is used to introduce the knowledge of the one or morestructure in in the optimization problem.vi. A metrology system according to clause v, wherein the processor isconfigured to define the optimization problem that comprises a gradientbased minimization of a loss function.vii. A metrology system according to clause vi, wherein the processor isconfigured to define the loss function that comprises a sum of a datafidelity function and a prior knowledge regularization function.viii. A metrology system according to any of the claims iii to vii,wherein the processor is configured to use complex numbers in thedefining of the optimization problem and the finding an optimum of theoptimization problemix. A metrology system according to any preceding clase, wherein thesensor is arranged in or near one of: a pupil plane of an opticalsystem, a plane that is conjugate with the pupil plane of the opticalsystem and an image plane of the optical system.x. A metrology system according to any of the clauses i to ix, whereinthe sensor is arranged in or near the far field of the one of morestructures on the substrate and the metrology system is configured toprovide free space propagation for radiation propagating from the one ormore structure on the substrate towards the sensor.xi. A metrology system according to any preceding clause, comprising anillumination system for illuminating the one or more structures on thesubstrate, the illumination system comprising a radiation source, theradiation source is one of a spatially coherent light source, aspatially incoherent light source and a spatially incoherent lightsource.xii. A metrology system according to any preceding clause, wherein thecharacteristics of the one or more structures on the substrate compriseat least one of

an overlay value of structures in different layers of the one or morestructures,

one or more geometrical parameter values of 2d or 3d structures of theone or more structures, for example, a critical dimension of the one ormore structures, a side wall angle of the one or more structures, anedge placement error of the one or more structures, or a local criticaldimension uniformity value of the one or more structures,

an operational parameter of an apparatus that manufactured the one ormore structures on the substrate, for example, a focus value relating toa focus of a lithographic apparatus that manufactured the one or morestructures, or a dose value relating to a dose being used by thelithographic apparatus that manufactured the one or more structures.

xiii. Method of determining a characteristic of one or more structureson a substrate, the method comprising

detecting characteristics of electromagnetic radiation that is reflectedor scattered by the one or more structures,

determining phase and amplitude of an electric field in the vicinity ofthe one or more structures on the substrate,

using the determined phase and amplitude of the electric field todetermine the characteristic.

Further embodiments are disclosed in the subsequent numbered clauses:

1. A metrology system for determining a characteristic of interestrelating to at least one structure on a substrate, the metrology systemcomprising:

-   -   a processor being configured to:        -   computationally determine phase and amplitude information            from a detected characteristic of scattered radiation.            2. A metrology system as defined in clause 1 wherein the            scattered radiation has been reflected or scattered by the            at least one structure as a result of illumination of said            at least one structure with illumination radiation in a            measurement acquisition.            3. A metrology system as defined in clause 1 or 2, the            processor being further configured to use the determined            phase and amplitude to determine the characteristic of            interest.            4. A metrology system as defined in any preceding clause,            wherein the processor is configured such that said            computationally determining phase and amplitude information            comprises computationally determining the phase and            amplitude of an electric and/or magnetic field and/or source            representing the at least one structure.            5. A metrology system as defined in clause 4, wherein the            processor is further configured to:

model the effect of interaction between the illumination radiation andthe at least one structure on the scattered radiation to obtain amodelled characteristic of the scattered radiation; and

optimize the phase and amplitude of the electric field so as to minimizethe difference between the modelled characteristic of the scatteredradiation and the detected characteristic of the scattered radiation.

6. A metrology system as defined in clause 5, wherein said model isoperable to model the effect of interaction between the illuminationradiation and the at least one structure as an dense array of smallelectrical and/or magnetic current dipoles, wherein, optionally, thedense array is an infinitely dense array, and wherein, optionally, thesmall electric and/or magnetic current dipoles are infinitesimal.7. A metrology system as defined in clause 6, wherein the dense array ofsmall electrical current dipoles are modelled on a two-dimensionalplane.8. A metrology system as defined in clause 5, 6 or 7, wherein themetrology system comprises one or more optical elements between the atleast one structure and a detection plane where the scattered radiationis detected, and the processor is configured to additionally model theeffect of said one or more optical elements on the scattered radiationto obtain the modelled characteristic of the scattered radiation.9. A metrology system as defined in any of clauses 5 to 8, wherein theprocessor is configured to use prior knowledge of the at least onestructure to optimize the phase and amplitude of the electric field.10. A metrology system as defined in clause 9, wherein the processor isconfigured to use at least a part of the prior knowledge of the at leastone structure as a regularization to and/or a constraint on theoptimization of the phase and amplitude of the electric field.11. A metrology system as defined in clause 10, wherein saidregularization and/or constraint comprises a total variation basedregularization and/or constraint.12. A metrology system as defined in clause 10, wherein saidregularization and/or constraint comprises a vector total variationbased regularization and/or constraint.13. A metrology system as defined in any of clauses 10 to 12, whereinsaid regularization and/or constraint imposes a coupling betweendifferent sets of values of the phase and amplitude information for theat least one structure, each set of values relating to a differentillumination condition.14. A metrology system as defined in clause 13, wherein the processor isoperable to computationally determine said different sets of values ofthe phase and amplitude information simultaneously thereby suppressingcoherent imaging artifacts such as image speckle and/or ringingartifacts.15. A metrology system according to any of clauses 10 to 14, wherein theprocessor is configured to:

define a loss function that describes the difference between themodelled characteristic of the scattered radiation and the detectedcharacteristic of the scattered radiation; and

minimize said loss function using at least a part of the prior knowledgeof the at least one structure as a regularization and/or constraint onsaid minimization.

16. A metrology system according to any of clauses 10 to 15, wherein theprocessor is configured to define a loss function that comprises a sumof a data fidelity function and a prior knowledge regularizationfunction, the data fidelity function describing the difference betweenthe modelled characteristic of the scattered radiation and the detectedcharacteristic of the scattered radiation which is to be minimized andthe prior knowledge regularization function being operable to performsaid regularization and/or constrain the minimization.17. A metrology system according to clause 16, wherein the processor isconfigured to define said minimization as a gradient method basedminimization of the loss function or a Newton's method basedminimization of the loss function.18. A metrology system according to clause 17, wherein the processor isconfigured to use the dominant singular value of the Jacobian matrixrelating to the prior knowledge regularization function, as the gradientmagnitude or an approximation thereof in the gradient based minimizationof the loss function.19. A metrology system according to clause 17, wherein the processor isconfigured to use a suitable function of the singular values and/or theeigenvalues of a Jacobian matrix relating to the prior knowledgeregularization function, in the gradient based minimization of the lossfunction.20. A metrology system according to any of clauses 16 to 19, wherein theprocessor is configured to:

model variation in refractive index of the at least one structureagainst wavelength and/or polarization and/or propagation direction todetermine a dispersion model of structure; and

use the prior knowledge regularization function on input coefficients ofsaid dispersion model.

21. A metrology system according to any of clauses 10 to 20, whereinsaid regularization and/or constraint comprises a minimum descriptionlength based regularization or constraint which imposes a bias againstcomplex solutions to the optimization.

22. A metrology system according to clause 21, wherein said minimumdescription length based regularization or constraint comprises aKolmogorov complexity based regularization and/or constraint.

23. A metrology system according to any of clauses 10 to 22, whereinsaid regularization or constraint comprises a matrix rank or nuclearnorm based regularization and/or constraint which imposes a low-rankconstraint or an approximation thereof on the solution to theoptimization.24. A metrology system according to any of clauses 10 to 23, wherein theprocessor is configured to use a search library to constrain solutionsto the optimization to only those which are a linear combination oflibrary images of the at least one structure comprised within the searchlibrary.25. A metrology system according to any of clauses 10 to 25, wherein theprocessor is configured such that said regularization is further basedon different sets of values of the phase and amplitude information forthe at least one structure, each set of values relating to a differentfocus setting, said metrology system comprising an electrical tunablelens for changing the focus settings without changing the relativedisplacement between structure and sensor.26. A metrology system according to any of clauses 5 to 25, wherein theprocessor is configured to use complex numbers in the defining andperformance of said optimization of the phase and amplitude of theelectric field.27. A metrology system according to any preceding clause, wherein theprocessor is further configured to simultaneously computationallydetermine the phase and amplitude information for multiple images of thetarget, each image corresponding to a different diffraction order.28. A metrology system according to any preceding clause, wherein theprocessor is further configured to computationally re-image themeasurement acquisition of the at least one structure subsequent to ameasurement to obtain at least one computationally re-imaged image.29. A metrology system according to clause 28, wherein saidcomputationally re-imaging the measurement acquisition comprises one ormore of:

digitally altering one or more illumination characteristics, which mayinclude: the illumination coherence or coherence at target level, theillumination profile, the illumination spectrum shape, the illuminationpolarization, and/or digitally imposing polarization and/or apodization;

digitally altering one or more characteristics of the detection optics,which may include: changing a numerical aperture of the detectionoptics, changing any other characteristic of an optical component of thedetection optics, changing an aberration characteristic of an opticalcomponent of the detection optics, filtering in the detection optics;

digitally altering a focus setting of an image.

30. A metrology system according to clause 28 or 29, wherein theprocessor is further configured to:

computationally re-image the measurement acquisition of the at least onestructure subsequent to a measurement for a plurality of differentvirtual settings of a parameter to obtain a plurality of computationallyre-imaged images, and average the plurality of computationally re-imagedimages to obtain an averaged computationally re-imaged image.

31. A metrology system according to claim 30, wherein the processor isfurther configured to apply a positive, zero or negative weight to eachof the computationally re-imaged images to obtain a weighted averagedcomputationally re-imaged image, the weights for all the computationallyre-imaged images having a sum of one.32. A metrology system according to any preceding clause, wherein theprocessor uses a feasibility problem approach, such as for example aGerchberg-Saxton algorithm, to determine the phase and the amplitude.33. A metrology system according to any preceding clause, furthercomprising one or more sensors for detecting characteristics of saidscattered radiation subsequent to it having been reflected or scatteredby the at least one structure.34. A metrology system according to claim 33, wherein the metrologyapparatus is configured to prevent a transmission of a zerothdiffraction order of said scattered radiation towards the one or moresensors.35. A metrology system according to clause 33 or 34, wherein at leastone of said one or more sensors is arranged in or near one of: a pupilplane of an optical system or a plane that is conjugate with the pupilplane of the optical system.36. A metrology system according to clause 33 or 35, wherein at leastone of said one or more sensors is arranged in an image plane of theoptical system or a plane that is conjugate with the image plane.37. A metrology system according to clause 33, 34, 35 or 36, wherein atleast one of said one or more sensors is arranged in or near the farfield of the at least one structure and the metrology system isconfigured to provide free space propagation for the scattered radiationpropagating from the at least one structure towards the sensor.38. A metrology system according to any of clauses 1 to 35, wherein themetrology system is operable to perform a measurement acquisition on theat least one structure with a plurality of different levels of focus;and

use the results of each of these measurement acquisitions to determinethe characteristic of interest.

39. A metrology system according to any preceding clause, wherein theprocessor is further configured to computationally refocus themeasurement acquisition of the at least one structure subsequent to ameasurement.

40. A metrology system according to any preceding clause, comprising anillumination system for illuminating the at least one structure on thesubstrate, the illumination system comprising a radiation source, theradiation source being one of: a spatially coherent light source, aspatially incoherent light source and a spatially partially coherentlight source.41. A metrology system according to any preceding clause, configured todetermine aberration and/or speckle perturbations in an illuminationprofile of the illumination radiation on the at least one structure; and

wherein the processor is configured to use the determined aberrationand/or speckle perturbations in determining the characteristic ofinterest.

42. A metrology system according to any preceding clause, configured todetermine aberration perturbations in the detection optics; and

wherein the processor is configured to use the determined aberrationperturbations in determining the characteristic of interest.

43. A metrology system according to any preceding clause, wherein theprocessor is further configured to computationally determine the phaseand amplitude for different illumination conditions than those actuallyused in performing a measurement acquisition.44. A metrology system according to any preceding clause, wherein thecharacteristic of interest comprises one or more of:

-   -   an overlay value relating to a misalignment of sub-structures in        different layers of the at least one structure,    -   a focus value relating to a focus of a lithographic apparatus        that manufactured the at least one structure, and    -   a dose value relating to a dose being used by the lithographic        apparatus that manufactured the at least one structure.        45. A metrology system according to any of clauses 1 to 43,        wherein the characteristic of interest comprises one or more        geometrical parameter values of the at least one structure,        wherein said one or more geometrical parameter values includes        one or more of: a critical dimension relating to the at least        one structure, a side wall angle relating to the at least one        structure, an edge placement error relating to the at least one        structure, or a local critical dimension uniformity value        relating to the at least one structure.        46. A metrology system according to any preceding clause,        wherein the metrology system is operable to perform dark field        measurements such that a zeroth order of the scattered radiation        is completely or partially blocked, and the phase information        and characteristic of interest is determined from at least one        pair of higher positive and negative diffraction orders of the        scattered radiation.        47. A method of determining a characteristic of interest        relating to at least one structure on a substrate, method        comprising:

computationally determining phase and amplitude information from adetected characteristic of scattered radiation.

48. A method as defined in clause 47, wherein the scattered radiationhas been reflected or scattered by the at least one structure as aresult of illumination of said at least one structure with illuminationradiation in a measurement acquisition.

49. A method as defined in clause 47 or 48 further comprising using thedetermined phase and amplitude to determine the characteristic ofinterest.

50. A method as defined in clause 47, 48 or 49, wherein computationallydetermining phase and amplitude information comprises computationallydetermining the phase and amplitude of an electric and/or magnetic fieldrepresenting the at least one structure.

51. A method as defined in clause 50, comprising:

modeling the effect of interaction between the illumination radiationand the at least one structure on the scattered radiation to obtain amodelled characteristic of the scattered radiation; and

optimizing the phase and amplitude of the electric field so as tominimize the difference between the modelled characteristic of thescattered radiation and the detected characteristic of the scatteredradiation.

52. A method as defined in clause 51, comprising modeling the at leastone structure as an dense array of small electrical and/or magneticcurrent dipoles on a two-dimensional plane, wherein, optionally, thedense array is an infinitely dense array, and wherein, optionally, thesmall electric and/or magnetic current dipoles are infinitesimal.53. A method as defined in clause 51 or 52, wherein there is one or moreoptical elements between the at least one structure and a detectionplane where the scattered radiation is detected, the method comprisingadditionally modeling the effect of said one or more optical elements onthe scattered radiation to obtain the modelled characteristic of thescattered radiation.54. A method as defined in any of clauses 51 to 53, comprising usingprior knowledge of the at least one structure to optimize the phase andamplitude of the electric field.55. A method as defined in clause 54, comprising using the priorknowledge of the at least one structure as a regularization to and/or aconstraint on the optimization of the phase and amplitude of theelectric field by:

defining a loss function that describes the difference between themodelled characteristic of the scattered radiation and the detectedcharacteristic of the scattered radiation; and

minimizing said loss function using the prior knowledge of the at leastone structure as the regularization and/or constraint on saidminimization.

56. A method as defined in clause 54 or 55 wherein said regularizationand/or constraint imposes a coupling between different sets of values ofthe phase and amplitude information for the at least one structure, eachset of values relating to a different illumination condition.57. A method according to any of clauses 54 to 56, comprising defining aloss function that comprises a sum of a data fidelity function and aprior knowledge regularization function, the data fidelity functiondescribing the difference between the modelled characteristic of thescattered radiation and the detected characteristic of the scatteredradiation which is to be minimized and the prior knowledgeregularization function being operable to perform said regularizationand/or constrain the minimization.58. A method according to any of clauses 54 to 57, wherein saidregularization and/or constraint comprises one or more of:

a minimum description length based regularization and/or constraintwhich imposes a bias against complex solutions to the optimization; or

a matrix rank or nuclear norm based regularization and/or constraintwhich imposes a low-rank constraint on the solution to the optimization.

59. A method according to any of clauses 51 to 58, comprising usecomplex numbers in the defining and performance of said optimization ofthe phase and amplitude of the electric field.

60. A method according to any of clauses 47 to 59, comprising sensingthe scattered radiation, wherein sensing is performed in or near one ormore of:

a pupil plane of an optical system or a plane that is conjugate with thepupil plane of the optical system;

an image plane of the optical system; and/or

in or near the far field of the at least one structure so as to providefree space propagation for the scattered radiation propagating from theat least one structure towards the sensor.

61. A method according to any of clauses 47 to 60, comprising:

performing a measurement acquisition on the at least one structure witha plurality of different levels of focus; and

using the results of each of these measurement acquisitions to determinethe characteristic of interest.

62. A method according to any of clauses 47 to 61, comprisingcomputationally refocusing the measurement acquisition of the at leastone structure subsequent to a measurement.

63. A method according to any of clauses 47 to 62, comprisingsimultaneously computationally determine the phase and amplitudeinformation for multiple images of the target, each image correspondingto a different diffraction order.

64. A method according to any of clauses 47 to 63, comprisingcomputationally determining the phase and amplitude for differentillumination conditions than those actually used in performing ameasurement acquisition.

65. A method according to any of clauses 47 to 64, comprisingcomputationally re-imaging the measurement acquisition of the at leastone structure subsequent to a measurement to obtain at least onecomputationally re-imaged image.

66. A method according to clause 65, wherein said computationallyre-imaging the measurement acquisition comprises one or more of:

digitally altering one or more illumination characteristics, which mayinclude: the illumination coherence, the target coherence, theillumination profile, the illumination spectrum shape, the illuminationpolarization, and/or digitally imposing polarization and/or apodization;

digitally altering one or more characteristics of the detection optics,which may include: changing a numerical aperture of the detectionoptics, changing any other characteristic of an optical component of thedetection optics, changing an aberration characteristic of an opticalcomponent of the detection optics, filtering in the detection optics;

digitally altering a focus setting of an image.

67. A method according to clause 64 or 65, comprising:

computationally re-imaging the measurement acquisition of the at leastone structure subsequent to a measurement for a plurality of differentvirtual settings of a parameter to obtain a plurality of computationallyre-imaged images, and

averaging the plurality of computationally re-imaged images to obtain anaveraged computationally re-imaged image.

68. A method according to clause 67, comprising applying a positive,zero or negative weight to each of the computationally re-imaged imagesto obtain a weighted averaged computationally re-imaged image, theweights for all the computationally re-imaged images having a sum ofone.69. A method according to any of clauses 47 to 68, wherein thecharacteristic of interest comprises one or more of:

-   -   an overlay value relating to a misalignment of sub-structures in        different layers of the at least one structure,    -   an overlay value relating to a misalignment of sub-structures in        the same layer of the at least one structure in a        multi-patterning process;    -   a focus value relating to a focus of a lithographic apparatus        that manufactured the at least one structure,    -   a dose value relating to a dose being used by the lithographic        apparatus that manufactured the at least one structure; and/or    -   one or more geometrical parameter values of the at least one        structure, wherein said one or more geometrical parameter values        includes one or more of: a critical dimension relating to the at        least one structure, a side wall angle relating to the at least        one structure, an edge placement error relating to the at least        one structure, or a local critical dimension uniformity value        relating to the at least one structure.        70. A non-transitory computer program product comprising        machine-readable instructions for causing a processor to cause        performance of the method of any of clauses 47 to 69.        71. A metrology apparatus for determining a characteristic of        interest relating to at least one structure on a substrate, the        metrology apparatus comprising at least one of    -   one or more radiation sources being operable of generating        radiation in an emission wavelength range that at least        partially overlaps with a wavelength range from 200 to 2000 nm,        or, optionally, the emission wavelength range at least overlaps        with half of the wavelength range from 200 nm to 2000 nm,    -   an optical system being operable of transmitting or reflecting        radiation in the wavelength range from 200 nm to 2000 nm, or,        optionally, the optical system being operable of transmitting or        reflecting radiation in at least half of the wavelength range        from 200 nm to 2000 nm, or, optionally, the optical system being        operable of transmitting or reflecting radiation in at least ¾        of the wavelength range from 200 nm to 2000 nm,    -   the optical system being operable to illuminate the structure        with a Numerical Aperture (NA) larger than 0.4, larger than 0.6,        larger than 0.7, or, optionally, larger than 0.8,    -   the optical system being operable to capture reflected and/or        scattered radiation with an detection optical sub-system and        wherein the detection optical sub-system has a Numerical        Aperture (NA) larger than 0.4, larger than 0.6, larger than 0.7,        or, optionally, larger than 0.8,    -   the optical system having an aberration larger than the used        illumination wavelength (        ) divided by 20,    -   the optical system being operable to illuminate the structure        with a Field of View (FoV) larger than 40 by 40 micrometer, or        optionally larger than 50 by 50 micrometer, or optionally larger        than 75 by 75 micrometer    -   the optical system having a transmission or reflectivity of at        least 25%, or optionally at least 75%, from the one or more        radiation sources towards the structure,    -   the optical system having a transmission or reflectivity of at        least 70%, or optionally at least 75%, for reflected and/or        scattered radiation from the structure towards one or more        sensors for recording characteristics of the reflected and/or        scattered radiation.        72. A metrology apparatus according to clause 71 further        comprising an optical system being configured to only transmit        one or more higher diffraction orders towards a sensor.        73. A metrology apparatus according to clause 72 wherein the        optical system further comprises a blocking means being operable        to block a zeroth diffraction order being reflected by the        structure on the substrate when the structure is illuminated        with radiation.        74. A metrology apparatus according to any one of the clauses 71        to 73, further comprising a processor being configured to        execute any one of the methods of clauses 47 to 69.        75. A metrology system according to any one of the clauses 1 to        46 comprising a metrology apparatus according to any one of the        clause 71 to 74.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications. Possible other applications include the manufactureof integrated optical systems, guidance and detection patterns formagnetic domain memories, flat-panel displays, liquid-crystal displays(LCDs), thin-film magnetic heads, etc.

Although specific reference may be made in this text to embodiments ofthe invention in the context of an inspection or metrology apparatus,embodiments of the invention may be used in other apparatus. Embodimentsof the invention may form part of a mask inspection apparatus, alithographic apparatus, or any apparatus that measures or processes anobject such as a wafer (or other substrate) or mask (or other patterningdevice).

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention, where the context allows, is notlimited to optical lithography and may be used in other applications,for example imprint lithography.

While the targets or target structures (more generally structures on asubstrate) described above are metrology target structures specificallydesigned and formed for the purposes of measurement, in otherembodiments, properties of interest may be measured on one or morestructures which are functional parts of devices formed on thesubstrate. Many devices have regular, grating-like structures. The termsstructure, target grating and target structure as used herein do notrequire that the structure has been provided specifically for themeasurement being performed. Further, pitch P of the metrology targetsis close to the resolution limit of the optical system of thescatterometer, but may be much larger than the dimension of typicalproduct features made by lithographic process in the target portions C.In practice the lines and/or spaces of the overlay gratings within thetarget structures may be made to include smaller structures similar indimension to the product features.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The descriptions above are intended to beillustrative, not limiting. Thus it will be apparent to one skilled inthe art that modifications may be made to the invention as describedwithout departing from the scope of the claims set out below.

The invention claimed is:
 1. A metrology apparatus for determining acharacteristic of interest relating to at least one structure on asubstrate, the metrology apparatus comprising: an illumination branchcomprising a radiation source and configured to direct illuminationradiation at the substrate; a detection branch comprising a detector andconfigured to detect scattered radiation from the at least one structureon the substrate; and a processor configured to: computationallydetermine phase and amplitude information from an electric field of thescattered radiation in a measurement acquisition, computationallyre-image the measurement acquisition of the at least one structuresubsequent to a measurement to obtain at least one computationallyre-imaged image, and computationally re-image the measurementacquisition of the at least one structure subsequent to a measurementfor a plurality of different virtual settings of a parameter to obtain aplurality of computationally re-imaged images, wherein no structuremodel is used to obtain the at least one computationally re-imagedimage.
 2. The metrology apparatus of claim 1, wherein the processor isfurther configured to use prior knowledge of the at least one structureto optimize the phase and amplitude of the electric field.
 3. Themetrology apparatus of claim 2, wherein the processor is furtherconfigured to use at least a part of the prior knowledge of the at leastone structure as a regularization to or a constraint on the optimizationof the phase and amplitude of the electric field.
 4. The metrologyapparatus of claim 1, wherein the processor is further configured to usethe determined phase and amplitude to determine a characteristic ofinterest.
 5. The metrology apparatus of claim 1, wherein computationallyre-imaging the measurement acquisition comprises digitally altering oneor more illumination characteristics.
 6. The metrology apparatus ofclaim 5, wherein the one or more illumination characteristics comprisesan illumination coherence, a target level illumination coherence, anillumination profile, an illumination spectrum shape, an illuminationpolarization, a digitally imposing illumination polarization, and/or anillumination apodization.
 7. The metrology apparatus of claim 1, whereincomputationally re-imaging the measurement acquisition comprisesdigitally altering one or more characteristics of detection optics ofthe detection branch.
 8. The metrology apparatus of claim 7, wherein theone or more characteristics of the detection optics comprises anumerical aperture of the detection optics, an optical component of thedetection optics, an aberration of the detection optics, a filter of thedetection optics, and/or a focus setting of the detection optics.
 9. Themetrology apparatus of claim 1, wherein the processor is furtherconfigured to average the plurality of computationally re-imaged imagesto obtain an averaged computationally re-imaged image.
 10. The metrologyapparatus of claim 9, wherein the processor is further configured toapply a positive, zero, or negative weight to each of thecomputationally re-imaged images to obtain a weighted averagedcomputationally re-imaged image, wherein the weights of all thecomputationally re-imaged images have a sum of one.
 11. A method ofdetermining a characteristic of interest relating to at least onestructure on a substrate, the method comprising: computationallydetermining phase and amplitude information from an electric field ofscattered radiation in a measurement acquisition; computationallyre-imaging the measurement acquisition of the at least one structuresubsequent to a measurement to obtain at least one computationallyre-imaged image; and computationally re-imaging the measurementacquisition of the at least one structure subsequent to a measurementfor a plurality of different virtual settings of a parameter to obtain aplurality of computationally re-imaged images, wherein no structuremodel is used to obtain the at least one computationally re-imagedimage.
 12. The method of claim 11, further comprising: using priorknowledge of the at least one structure to optimize the phase andamplitude of the electric field; and using at least a part of the priorknowledge of the at least one structure as a regularization to or aconstraint on the optimization of the phase and amplitude of theelectric field.
 13. The method of claim 11, further comprising using thedetermined phase and amplitude to determine a characteristic ofinterest.
 14. The method of claim 11, wherein the computationallyre-imaging the measurement acquisition comprises digitally altering oneor more illumination characteristics.
 15. The method of claim 14,wherein the one or more illumination characteristics comprises anillumination coherence, a target level illumination coherence, anillumination profile, an illumination spectrum shape, an illuminationpolarization, a digitally imposing illumination polarization, and/or anillumination apodization.
 16. The method of claim 11, wherein thecomputationally re-imaging the measurement acquisition comprisesdigitally altering one or more characteristics of detection optics. 17.The method of claim 16, wherein the one or more characteristics of thedetection optics comprises a numerical aperture of the detection optics,an optical component of the detection optics, an aberration of thedetection optics, a filter of the detection optics, and/or a focussetting of the detection optics.
 18. The method of claim 11, furthercomprising averaging the plurality of computationally re-imaged imagesto obtain an averaged computationally re-imaged image.
 19. The method ofclaim 18, further comprising applying a positive, zero, or negativeweight to each of the computationally re-imaged images to obtain aweighted averaged computationally re-imaged image, wherein the weightsof all the computationally re-imaged images have a sum of one.
 20. Anon-transitory computer program product comprising machine-readableinstructions for causing a processor to cause performance of the methodof claim 11.